diff --git a/CHANGELOG.md b/CHANGELOG.md
index 363cca8..bef2943 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,6 +7,15 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+## [1.2.5] - 2020-01-08
+
+### Fixed
+
+- Fixes [issues/5](https://github.com/patch-technology/patch-ruby/issues/5). Adds method `.openapi_nullable` which returns a set with all nullable properties. This method is referenced in `#to_hash` which returns a hash representation of the resource.
+- Removes non-deterministic integration spec that is creating an Estimate with a specific project-id. Since we do not control the test data (yet) and make queries against our Test projects, we cannot be sure that we will have offsets available for a project ID. Instead we can rely on Patch core specs to cover this.
+- Fixes a spec that was failing due to a Patch core change.
+- Add spec factories and refactor generated method specs to shared examples.
+
 ## [1.2.4] - 2020-10-14
 
 ### Added
diff --git a/Gemfile b/Gemfile
index b572ad0..76018c7 100644
--- a/Gemfile
+++ b/Gemfile
@@ -6,4 +6,5 @@ group :development, :test do
   gem 'rake', '~> 13.0.1'
   gem 'pry-byebug'
   gem 'rubocop'
+  gem 'factory_bot'
 end
diff --git a/Gemfile.lock b/Gemfile.lock
index 725b6e9..60f45a5 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -1,22 +1,34 @@
 PATH
   remote: .
   specs:
-    patch_ruby (1.2.4)
+    patch_ruby (1.2.5)
       json (~> 2.1, >= 2.1.0)
       typhoeus (~> 1.0, >= 1.0.1)
 
 GEM
   remote: https://rubygems.org/
   specs:
+    activesupport (6.1.1)
+      concurrent-ruby (~> 1.0, >= 1.0.2)
+      i18n (>= 1.6, < 2)
+      minitest (>= 5.1)
+      tzinfo (~> 2.0)
+      zeitwerk (~> 2.3)
     ast (2.4.1)
     byebug (11.1.3)
     coderay (1.1.3)
+    concurrent-ruby (1.1.7)
     diff-lcs (1.4.3)
     ethon (0.12.0)
       ffi (>= 1.3.0)
+    factory_bot (6.1.0)
+      activesupport (>= 5.0.0)
     ffi (1.13.1)
-    json (2.3.1)
+    i18n (1.8.7)
+      concurrent-ruby (~> 1.0)
+    json (2.4.1)
     method_source (1.0.0)
+    minitest (5.14.3)
     parallel (1.19.2)
     parser (2.7.1.4)
       ast (~> 2.4.1)
@@ -57,12 +69,16 @@ GEM
     ruby-progressbar (1.10.1)
     typhoeus (1.4.0)
       ethon (>= 0.9.0)
+    tzinfo (2.0.4)
+      concurrent-ruby (~> 1.0)
     unicode-display_width (1.7.0)
+    zeitwerk (2.4.2)
 
 PLATFORMS
   ruby
 
 DEPENDENCIES
+  factory_bot
   patch_ruby!
   pry-byebug
   rake (~> 13.0.1)
diff --git a/lib/patch_ruby/models/allocation.rb b/lib/patch_ruby/models/allocation.rb
index 78da294..47d40b8 100644
--- a/lib/patch_ruby/models/allocation.rb
+++ b/lib/patch_ruby/models/allocation.rb
@@ -14,10 +14,13 @@
 
 module Patch
   class Allocation
+    # A unique uid for the record. UIDs will be prepended by all_prod or all_test depending on the mode it was created in.
     attr_accessor :id
 
+    # A boolean indicating if this project is a production or test mode project.
     attr_accessor :production
 
+    # The amount (in grams) of allocated carbon offsets.
     attr_accessor :mass_g
 
     # Attribute mapping from ruby-style variable name to JSON key.
@@ -38,6 +41,13 @@ def self.openapi_types
       }
     end
 
+    # Set with nullable attributes.
+    def self.openapi_nullable
+      nullable_properties = Set.new
+
+      nullable_properties
+    end
+
     # Allows models with corresponding API classes to delegate API operations to those API classes
     # Exposes Model.operation_id which delegates to ModelsApi.new.operation_id
     # Eg. Order.create_order delegates to OrdersApi.new.create_order
diff --git a/lib/patch_ruby/models/create_mass_estimate_request.rb b/lib/patch_ruby/models/create_mass_estimate_request.rb
index 7c831aa..b101134 100644
--- a/lib/patch_ruby/models/create_mass_estimate_request.rb
+++ b/lib/patch_ruby/models/create_mass_estimate_request.rb
@@ -34,6 +34,13 @@ def self.openapi_types
       }
     end
 
+    # Set with nullable attributes.
+    def self.openapi_nullable
+      nullable_properties = Set.new
+
+      nullable_properties
+    end
+
     # Allows models with corresponding API classes to delegate API operations to those API classes
     # Exposes Model.operation_id which delegates to ModelsApi.new.operation_id
     # Eg. Order.create_order delegates to OrdersApi.new.create_order
diff --git a/lib/patch_ruby/models/create_order_request.rb b/lib/patch_ruby/models/create_order_request.rb
index 45a1c79..ed3eff5 100644
--- a/lib/patch_ruby/models/create_order_request.rb
+++ b/lib/patch_ruby/models/create_order_request.rb
@@ -42,6 +42,13 @@ def self.openapi_types
       }
     end
 
+    # Set with nullable attributes.
+    def self.openapi_nullable
+      nullable_properties = Set.new
+
+      nullable_properties
+    end
+
     # Allows models with corresponding API classes to delegate API operations to those API classes
     # Exposes Model.operation_id which delegates to ModelsApi.new.operation_id
     # Eg. Order.create_order delegates to OrdersApi.new.create_order
diff --git a/lib/patch_ruby/models/create_preference_request.rb b/lib/patch_ruby/models/create_preference_request.rb
index 01c6920..d069ac7 100644
--- a/lib/patch_ruby/models/create_preference_request.rb
+++ b/lib/patch_ruby/models/create_preference_request.rb
@@ -30,6 +30,13 @@ def self.openapi_types
       }
     end
 
+    # Set with nullable attributes.
+    def self.openapi_nullable
+      nullable_properties = Set.new
+
+      nullable_properties
+    end
+
     # Allows models with corresponding API classes to delegate API operations to those API classes
     # Exposes Model.operation_id which delegates to ModelsApi.new.operation_id
     # Eg. Order.create_order delegates to OrdersApi.new.create_order
diff --git a/lib/patch_ruby/models/error_response.rb b/lib/patch_ruby/models/error_response.rb
index e9f70c8..dd0ff95 100644
--- a/lib/patch_ruby/models/error_response.rb
+++ b/lib/patch_ruby/models/error_response.rb
@@ -38,6 +38,15 @@ def self.openapi_types
       }
     end
 
+    # Set with nullable attributes.
+    def self.openapi_nullable
+      nullable_properties = Set.new
+
+      nullable_properties.add("data")
+
+      nullable_properties
+    end
+
     # Allows models with corresponding API classes to delegate API operations to those API classes
     # Exposes Model.operation_id which delegates to ModelsApi.new.operation_id
     # Eg. Order.create_order delegates to OrdersApi.new.create_order
diff --git a/lib/patch_ruby/models/estimate.rb b/lib/patch_ruby/models/estimate.rb
index eaf55bc..aebe3fa 100644
--- a/lib/patch_ruby/models/estimate.rb
+++ b/lib/patch_ruby/models/estimate.rb
@@ -14,12 +14,16 @@
 
 module Patch
   class Estimate
+    # A unique uid for the record. UIDs will be prepended by est_prod or est_test depending on the mode it was created in.
     attr_accessor :id
 
+    # A boolean indicating if this estimate is a production or test mode estimate.
     attr_accessor :production
 
+    # The type of estimate. Currently mass is the only supported value.
     attr_accessor :type
 
+    # An object returning the order associated with this estimate. See the [Order section](/?id=orders) for the full schema.
     attr_accessor :order
 
     # Attribute mapping from ruby-style variable name to JSON key.
@@ -42,6 +46,15 @@ def self.openapi_types
       }
     end
 
+    # Set with nullable attributes.
+    def self.openapi_nullable
+      nullable_properties = Set.new
+
+      nullable_properties.add("order")
+
+      nullable_properties
+    end
+
     # Allows models with corresponding API classes to delegate API operations to those API classes
     # Exposes Model.operation_id which delegates to ModelsApi.new.operation_id
     # Eg. Order.create_order delegates to OrdersApi.new.create_order
@@ -226,7 +239,7 @@ def to_hash
           is_nullable = self.class.openapi_nullable.include?(attr)
           next if !is_nullable || (is_nullable && !instance_variable_defined?(:"@#{attr}"))
         end
-        
+
         hash[param] = _to_hash(value)
       end
       hash
diff --git a/lib/patch_ruby/models/estimate_list_response.rb b/lib/patch_ruby/models/estimate_list_response.rb
index 6866d42..8762d98 100644
--- a/lib/patch_ruby/models/estimate_list_response.rb
+++ b/lib/patch_ruby/models/estimate_list_response.rb
@@ -42,6 +42,15 @@ def self.openapi_types
       }
     end
 
+    # Set with nullable attributes.
+    def self.openapi_nullable
+      nullable_properties = Set.new
+
+      nullable_properties.add("error")
+
+      nullable_properties
+    end
+
     # Allows models with corresponding API classes to delegate API operations to those API classes
     # Exposes Model.operation_id which delegates to ModelsApi.new.operation_id
     # Eg. Order.create_order delegates to OrdersApi.new.create_order
diff --git a/lib/patch_ruby/models/estimate_response.rb b/lib/patch_ruby/models/estimate_response.rb
index aaed5d1..bc36f5f 100644
--- a/lib/patch_ruby/models/estimate_response.rb
+++ b/lib/patch_ruby/models/estimate_response.rb
@@ -38,6 +38,15 @@ def self.openapi_types
       }
     end
 
+    # Set with nullable attributes.
+    def self.openapi_nullable
+      nullable_properties = Set.new
+
+      nullable_properties.add("error")
+
+      nullable_properties
+    end
+
     # Allows models with corresponding API classes to delegate API operations to those API classes
     # Exposes Model.operation_id which delegates to ModelsApi.new.operation_id
     # Eg. Order.create_order delegates to OrdersApi.new.create_order
diff --git a/lib/patch_ruby/models/meta_index_object.rb b/lib/patch_ruby/models/meta_index_object.rb
index fe2cc06..5eac744 100644
--- a/lib/patch_ruby/models/meta_index_object.rb
+++ b/lib/patch_ruby/models/meta_index_object.rb
@@ -34,6 +34,17 @@ def self.openapi_types
       }
     end
 
+    # Set with nullable attributes.
+    def self.openapi_nullable
+      nullable_properties = Set.new
+
+      nullable_properties.add("prev_page")
+
+      nullable_properties.add("next_page")
+
+      nullable_properties
+    end
+
     # Allows models with corresponding API classes to delegate API operations to those API classes
     # Exposes Model.operation_id which delegates to ModelsApi.new.operation_id
     # Eg. Order.create_order delegates to OrdersApi.new.create_order
diff --git a/lib/patch_ruby/models/order.rb b/lib/patch_ruby/models/order.rb
index 55b5bb1..5418e41 100644
--- a/lib/patch_ruby/models/order.rb
+++ b/lib/patch_ruby/models/order.rb
@@ -14,22 +14,31 @@
 
 module Patch
   class Order
+    # A unique uid for the record. UIDs will be prepended by ord_prod or ord_test depending on the mode it was created in.
     attr_accessor :id
 
+    # The amount of carbon offsets in grams purchased through this order.
     attr_accessor :mass_g
 
+    # A boolean indicating if this order is a production or test mode order.
     attr_accessor :production
 
+    # The current state of the order.
     attr_accessor :state
 
+    # The current state of the allocated carbon offsets of the order.
     attr_accessor :allocation_state
 
+    # The total price in cents USD of the carbon offsets purchased through this order.
     attr_accessor :price_cents_usd
 
+    # The Patch Fee in cents USD for this order.
     attr_accessor :patch_fee_cents_usd
 
+    # An array containing the inventory allocations for this order.
     attr_accessor :allocations
 
+    # An optional JSON object containing metadata for this order.
     attr_accessor :metadata
 
     class EnumAttributeValidator
@@ -84,6 +93,17 @@ def self.openapi_types
       }
     end
 
+    # Set with nullable attributes.
+    def self.openapi_nullable
+      nullable_properties = Set.new
+
+      nullable_properties.add("price_cents_usd")
+
+      nullable_properties.add("patch_fee_cents_usd")
+
+      nullable_properties
+    end
+
     # Allows models with corresponding API classes to delegate API operations to those API classes
     # Exposes Model.operation_id which delegates to ModelsApi.new.operation_id
     # Eg. Order.create_order delegates to OrdersApi.new.create_order
diff --git a/lib/patch_ruby/models/order_list_response.rb b/lib/patch_ruby/models/order_list_response.rb
index 9d889b0..5786b7c 100644
--- a/lib/patch_ruby/models/order_list_response.rb
+++ b/lib/patch_ruby/models/order_list_response.rb
@@ -42,6 +42,15 @@ def self.openapi_types
       }
     end
 
+    # Set with nullable attributes.
+    def self.openapi_nullable
+      nullable_properties = Set.new
+
+      nullable_properties.add("error")
+
+      nullable_properties
+    end
+
     # Allows models with corresponding API classes to delegate API operations to those API classes
     # Exposes Model.operation_id which delegates to ModelsApi.new.operation_id
     # Eg. Order.create_order delegates to OrdersApi.new.create_order
diff --git a/lib/patch_ruby/models/order_response.rb b/lib/patch_ruby/models/order_response.rb
index 4af787c..f42405a 100644
--- a/lib/patch_ruby/models/order_response.rb
+++ b/lib/patch_ruby/models/order_response.rb
@@ -38,6 +38,15 @@ def self.openapi_types
       }
     end
 
+    # Set with nullable attributes.
+    def self.openapi_nullable
+      nullable_properties = Set.new
+
+      nullable_properties.add("error")
+
+      nullable_properties
+    end
+
     # Allows models with corresponding API classes to delegate API operations to those API classes
     # Exposes Model.operation_id which delegates to ModelsApi.new.operation_id
     # Eg. Order.create_order delegates to OrdersApi.new.create_order
diff --git a/lib/patch_ruby/models/photo.rb b/lib/patch_ruby/models/photo.rb
index fb9671b..35c7bf2 100644
--- a/lib/patch_ruby/models/photo.rb
+++ b/lib/patch_ruby/models/photo.rb
@@ -16,20 +16,31 @@ module Patch
   class Photo
     attr_accessor :url
 
+    attr_accessor :id
+
     # Attribute mapping from ruby-style variable name to JSON key.
     def self.attribute_map
       {
-        :'url' => :'url'
+        :'url' => :'url',
+        :'id' => :'id'
       }
     end
 
     # Attribute type mapping.
     def self.openapi_types
       {
-        :'url' => :'String'
+        :'url' => :'String',
+        :'id' => :'String'
       }
     end
 
+    # Set with nullable attributes.
+    def self.openapi_nullable
+      nullable_properties = Set.new
+
+      nullable_properties
+    end
+
     # Allows models with corresponding API classes to delegate API operations to those API classes
     # Exposes Model.operation_id which delegates to ModelsApi.new.operation_id
     # Eg. Order.create_order delegates to OrdersApi.new.create_order
@@ -69,6 +80,10 @@ def list_invalid_properties
         invalid_properties.push('invalid value for "url", url cannot be nil.')
       end
 
+      if @id.nil?
+        invalid_properties.push('invalid value for "id", id cannot be nil.')
+      end
+
       invalid_properties
     end
 
@@ -76,6 +91,7 @@ def list_invalid_properties
     # @return true if the model is valid
     def valid?
       return false if @url.nil?
+      return false if @id.nil?
       true
     end
 
@@ -84,7 +100,8 @@ def valid?
     def ==(o)
       return true if self.equal?(o)
       self.class == o.class &&
-          url == o.url
+          url == o.url &&
+          id == o.id
     end
 
     # @see the `==` method
@@ -96,7 +113,7 @@ def eql?(o)
     # Calculates hash code according to all attributes.
     # @return [Integer] Hash code
     def hash
-      [url].hash
+      [url, id].hash
     end
 
     # Builds the object from hash
diff --git a/lib/patch_ruby/models/preference.rb b/lib/patch_ruby/models/preference.rb
index 10bea26..48678c6 100644
--- a/lib/patch_ruby/models/preference.rb
+++ b/lib/patch_ruby/models/preference.rb
@@ -14,10 +14,13 @@
 
 module Patch
   class Preference
+    # A unique uid for the record. UIDs will be prepended by pre_prod or pre_test depending on the mode it was created in.
     attr_accessor :id
 
+    # Percentage of total purchased offsets that should go to a project. Default is 100%.
     attr_accessor :allocation_percentage
 
+    # An object returning the Project record this Preference is associated with. See the [Project section](/?id=projects) for the full schema.
     attr_accessor :project
 
     # Attribute mapping from ruby-style variable name to JSON key.
@@ -38,6 +41,13 @@ def self.openapi_types
       }
     end
 
+    # Set with nullable attributes.
+    def self.openapi_nullable
+      nullable_properties = Set.new
+
+      nullable_properties
+    end
+
     # Allows models with corresponding API classes to delegate API operations to those API classes
     # Exposes Model.operation_id which delegates to ModelsApi.new.operation_id
     # Eg. Order.create_order delegates to OrdersApi.new.create_order
diff --git a/lib/patch_ruby/models/preference_list_response.rb b/lib/patch_ruby/models/preference_list_response.rb
index fc8c29a..94ff4fc 100644
--- a/lib/patch_ruby/models/preference_list_response.rb
+++ b/lib/patch_ruby/models/preference_list_response.rb
@@ -42,6 +42,15 @@ def self.openapi_types
       }
     end
 
+    # Set with nullable attributes.
+    def self.openapi_nullable
+      nullable_properties = Set.new
+
+      nullable_properties.add("error")
+
+      nullable_properties
+    end
+
     # Allows models with corresponding API classes to delegate API operations to those API classes
     # Exposes Model.operation_id which delegates to ModelsApi.new.operation_id
     # Eg. Order.create_order delegates to OrdersApi.new.create_order
diff --git a/lib/patch_ruby/models/preference_response.rb b/lib/patch_ruby/models/preference_response.rb
index eb9c7ff..b3af523 100644
--- a/lib/patch_ruby/models/preference_response.rb
+++ b/lib/patch_ruby/models/preference_response.rb
@@ -38,6 +38,15 @@ def self.openapi_types
       }
     end
 
+    # Set with nullable attributes.
+    def self.openapi_nullable
+      nullable_properties = Set.new
+
+      nullable_properties.add("error")
+
+      nullable_properties
+    end
+
     # Allows models with corresponding API classes to delegate API operations to those API classes
     # Exposes Model.operation_id which delegates to ModelsApi.new.operation_id
     # Eg. Order.create_order delegates to OrdersApi.new.create_order
diff --git a/lib/patch_ruby/models/project.rb b/lib/patch_ruby/models/project.rb
index 0ce275b..c39529e 100644
--- a/lib/patch_ruby/models/project.rb
+++ b/lib/patch_ruby/models/project.rb
@@ -14,26 +14,37 @@
 
 module Patch
   class Project
+    # A unique uid for the record. UIDs will be prepended by pro_prod or pro_test depending on the mode it was created in.
     attr_accessor :id
 
+    # A boolean indicating if this project is a production or test mode project.
     attr_accessor :production
 
+    # The name of the project.
     attr_accessor :name
 
+    # The description of the project.
     attr_accessor :description
 
+    # The type of carbon removal project, currently available project types are Biomass, Dac, Forestry, Mineralization, Ocean, Soil.
     attr_accessor :type
 
+    # The country of origin of the project.
     attr_accessor :country
 
+    # The name of the project developer.
     attr_accessor :developer
 
+    # An array of URLs for photos of the project.
     attr_accessor :photos
 
+    # The average price per tonne in USD cents for carbon offsets supplied by this project.
     attr_accessor :average_price_per_tonne_cents_usd
 
+    # The remaining mass in grams available for purchase for this project.
     attr_accessor :remaining_mass_g
 
+    # An object returning the Standard associated with this project.
     attr_accessor :standard
 
     class EnumAttributeValidator
@@ -92,6 +103,17 @@ def self.openapi_types
       }
     end
 
+    # Set with nullable attributes.
+    def self.openapi_nullable
+      nullable_properties = Set.new
+
+      nullable_properties.add("photos")
+
+      nullable_properties.add("standard")
+
+      nullable_properties
+    end
+
     # Allows models with corresponding API classes to delegate API operations to those API classes
     # Exposes Model.operation_id which delegates to ModelsApi.new.operation_id
     # Eg. Order.create_order delegates to OrdersApi.new.create_order
diff --git a/lib/patch_ruby/models/project_list_response.rb b/lib/patch_ruby/models/project_list_response.rb
index ca207a8..c3ccb92 100644
--- a/lib/patch_ruby/models/project_list_response.rb
+++ b/lib/patch_ruby/models/project_list_response.rb
@@ -42,6 +42,15 @@ def self.openapi_types
       }
     end
 
+    # Set with nullable attributes.
+    def self.openapi_nullable
+      nullable_properties = Set.new
+
+      nullable_properties.add("error")
+
+      nullable_properties
+    end
+
     # Allows models with corresponding API classes to delegate API operations to those API classes
     # Exposes Model.operation_id which delegates to ModelsApi.new.operation_id
     # Eg. Order.create_order delegates to OrdersApi.new.create_order
diff --git a/lib/patch_ruby/models/project_response.rb b/lib/patch_ruby/models/project_response.rb
index 3dfa634..96cb48b 100644
--- a/lib/patch_ruby/models/project_response.rb
+++ b/lib/patch_ruby/models/project_response.rb
@@ -38,6 +38,15 @@ def self.openapi_types
       }
     end
 
+    # Set with nullable attributes.
+    def self.openapi_nullable
+      nullable_properties = Set.new
+
+      nullable_properties.add("error")
+
+      nullable_properties
+    end
+
     # Allows models with corresponding API classes to delegate API operations to those API classes
     # Exposes Model.operation_id which delegates to ModelsApi.new.operation_id
     # Eg. Order.create_order delegates to OrdersApi.new.create_order
diff --git a/lib/patch_ruby/models/standard.rb b/lib/patch_ruby/models/standard.rb
index d99883e..04a9cf1 100644
--- a/lib/patch_ruby/models/standard.rb
+++ b/lib/patch_ruby/models/standard.rb
@@ -14,10 +14,13 @@
 
 module Patch
   class Standard
+    # The standard type.
     attr_accessor :type
 
+    # The acronym for the standard.
     attr_accessor :acronym
 
+    # The description of the standard.
     attr_accessor :description
 
     # Attribute mapping from ruby-style variable name to JSON key.
@@ -38,6 +41,13 @@ def self.openapi_types
       }
     end
 
+    # Set with nullable attributes.
+    def self.openapi_nullable
+      nullable_properties = Set.new
+
+      nullable_properties
+    end
+
     # Allows models with corresponding API classes to delegate API operations to those API classes
     # Exposes Model.operation_id which delegates to ModelsApi.new.operation_id
     # Eg. Order.create_order delegates to OrdersApi.new.create_order
diff --git a/lib/patch_ruby/version.rb b/lib/patch_ruby/version.rb
index 0260ec8..dbdffe7 100644
--- a/lib/patch_ruby/version.rb
+++ b/lib/patch_ruby/version.rb
@@ -11,5 +11,5 @@
 =end
 
 module Patch
-  VERSION = '1.2.4'
+  VERSION = '1.2.5'
 end
diff --git a/spec/factories/allocations.rb b/spec/factories/allocations.rb
new file mode 100644
index 0000000..ac14f80
--- /dev/null
+++ b/spec/factories/allocations.rb
@@ -0,0 +1,7 @@
+FactoryBot.define do
+  factory :allocation, class: Patch::Allocation do
+    sequence(:id) { |n| n }
+    production { false }
+    mass_g { 100 }
+  end
+end
diff --git a/spec/factories/create_mass_estimate_requests.rb b/spec/factories/create_mass_estimate_requests.rb
new file mode 100644
index 0000000..c86faa7
--- /dev/null
+++ b/spec/factories/create_mass_estimate_requests.rb
@@ -0,0 +1,6 @@
+FactoryBot.define do
+  factory :create_mass_estimate_request, class: Patch::CreateMassEstimateRequest do
+    sequence(:project_id) { |n| n }
+    mass_g { 100 }
+  end
+end
diff --git a/spec/factories/create_order_requests.rb b/spec/factories/create_order_requests.rb
new file mode 100644
index 0000000..eae587e
--- /dev/null
+++ b/spec/factories/create_order_requests.rb
@@ -0,0 +1,8 @@
+FactoryBot.define do
+  factory :create_order_request, class: Patch::CreateOrderRequest do
+    sequence(:project_id) { |n| n }
+    mass_g { 100 }
+    total_price_cents_usd { 100 }
+    metadata { {} }
+  end
+end
diff --git a/spec/factories/create_preference_requests.rb b/spec/factories/create_preference_requests.rb
new file mode 100644
index 0000000..7dbce3f
--- /dev/null
+++ b/spec/factories/create_preference_requests.rb
@@ -0,0 +1,5 @@
+FactoryBot.define do
+  factory :create_preference_request, class: Patch::CreatePreferenceRequest do
+    sequence(:project_id) { |n| n }
+  end
+end
diff --git a/spec/factories/error_responses.rb b/spec/factories/error_responses.rb
new file mode 100644
index 0000000..0ff64f5
--- /dev/null
+++ b/spec/factories/error_responses.rb
@@ -0,0 +1,7 @@
+FactoryBot.define do
+  factory :error_response, class: Patch::ErrorResponse do
+    success { true }
+    error { {} }
+    data { {} }
+  end
+end
diff --git a/spec/factories/estimate_list_responses.rb b/spec/factories/estimate_list_responses.rb
new file mode 100644
index 0000000..9aa3af6
--- /dev/null
+++ b/spec/factories/estimate_list_responses.rb
@@ -0,0 +1,8 @@
+FactoryBot.define do
+  factory :estimate_list_response, class: Patch::EstimateListResponse do
+    success { true }
+    error { {} }
+    data { {} }
+    meta { {} }
+  end
+end
diff --git a/spec/factories/estimate_responses.rb b/spec/factories/estimate_responses.rb
new file mode 100644
index 0000000..ff27c80
--- /dev/null
+++ b/spec/factories/estimate_responses.rb
@@ -0,0 +1,7 @@
+FactoryBot.define do
+  factory :estimate_response, class: Patch::EstimateResponse do
+    success { true }
+    error { {} }
+    data { {} }
+  end
+end
diff --git a/spec/factories/estimates.rb b/spec/factories/estimates.rb
new file mode 100644
index 0000000..afc8033
--- /dev/null
+++ b/spec/factories/estimates.rb
@@ -0,0 +1,8 @@
+FactoryBot.define do
+  factory :estimate, class: Patch::Estimate do
+    sequence(:id) { |n| n }
+    association :order
+    production { false }
+    type { "mass" }
+  end
+end
diff --git a/spec/factories/meta_index_objects.rb b/spec/factories/meta_index_objects.rb
new file mode 100644
index 0000000..a504cf0
--- /dev/null
+++ b/spec/factories/meta_index_objects.rb
@@ -0,0 +1,6 @@
+FactoryBot.define do
+  factory :meta_index_object, class: Patch::MetaIndexObject do
+    sequence(:next_page) { |n| n + 1 }
+    sequence(:prev_page) { |n| n - 1 }
+  end
+end
diff --git a/spec/factories/order_list_responses.rb b/spec/factories/order_list_responses.rb
new file mode 100644
index 0000000..b1f6b46
--- /dev/null
+++ b/spec/factories/order_list_responses.rb
@@ -0,0 +1,8 @@
+FactoryBot.define do
+  factory :order_list_response, class: Patch::OrderListResponse do
+    success { true }
+    error { {} }
+    data { {} }
+    meta { {} }
+  end
+end
diff --git a/spec/factories/order_responses.rb b/spec/factories/order_responses.rb
new file mode 100644
index 0000000..3bde446
--- /dev/null
+++ b/spec/factories/order_responses.rb
@@ -0,0 +1,7 @@
+FactoryBot.define do
+  factory :order_response, class: Patch::OrderResponse do
+    success { true }
+    error { {} }
+    data { {} }
+  end
+end
diff --git a/spec/factories/orders.rb b/spec/factories/orders.rb
new file mode 100644
index 0000000..1b4457f
--- /dev/null
+++ b/spec/factories/orders.rb
@@ -0,0 +1,12 @@
+FactoryBot.define do
+  factory :order, class: Patch::Order do
+    sequence(:id) { |n| n }
+    mass_g { 1_000 }
+    production { false }
+    state { "draft" }
+    allocation_state { "pending" }
+    price_cents_usd { 500 }
+    patch_fee_cents_usd { 5 }
+    metadata { {} }
+  end
+end
diff --git a/spec/factories/preference_list_responses.rb b/spec/factories/preference_list_responses.rb
new file mode 100644
index 0000000..f163ade
--- /dev/null
+++ b/spec/factories/preference_list_responses.rb
@@ -0,0 +1,8 @@
+FactoryBot.define do
+  factory :preference_list_response, class: Patch::PreferenceListResponse do
+    success { true }
+    error { {} }
+    data { {} }
+    meta { {} }
+  end
+end
diff --git a/spec/factories/preference_responses.rb b/spec/factories/preference_responses.rb
new file mode 100644
index 0000000..1fcb2a3
--- /dev/null
+++ b/spec/factories/preference_responses.rb
@@ -0,0 +1,7 @@
+FactoryBot.define do
+  factory :preference_response, class: Patch::PreferenceResponse do
+    success { true }
+    error { {} }
+    data { {} }
+  end
+end
diff --git a/spec/factories/preferences.rb b/spec/factories/preferences.rb
new file mode 100644
index 0000000..7489ade
--- /dev/null
+++ b/spec/factories/preferences.rb
@@ -0,0 +1,7 @@
+FactoryBot.define do
+  factory :preference, class: Patch::Preference do
+    sequence(:id) { |n| n }
+    association :project
+    allocation_percentage { 1 }
+  end
+end
diff --git a/spec/factories/project_list_responses.rb b/spec/factories/project_list_responses.rb
new file mode 100644
index 0000000..4d06f06
--- /dev/null
+++ b/spec/factories/project_list_responses.rb
@@ -0,0 +1,8 @@
+FactoryBot.define do
+  factory :project_list_response, class: Patch::ProjectListResponse do
+    success { true }
+    error { {} }
+    data { {} }
+    meta { {} }
+  end
+end
diff --git a/spec/factories/project_responses.rb b/spec/factories/project_responses.rb
new file mode 100644
index 0000000..331be37
--- /dev/null
+++ b/spec/factories/project_responses.rb
@@ -0,0 +1,7 @@
+FactoryBot.define do
+  factory :project_response, class: Patch::ProjectResponse do
+    success { true }
+    error { {} }
+    data { {} }
+  end
+end
diff --git a/spec/factories/projects.rb b/spec/factories/projects.rb
new file mode 100644
index 0000000..dea7ab7
--- /dev/null
+++ b/spec/factories/projects.rb
@@ -0,0 +1,15 @@
+FactoryBot.define do
+  factory :project, class: Patch::Project do
+    sequence(:id) { |n| n }
+    production { false }
+    name { "New Project" }
+    description { "New Descirption" }
+    type { "biomass" }
+    country { "DK" }
+    developer { "Danish Developer" }
+    photos { [] }
+    average_price_per_tonne_cents_usd { 120 }
+    remaining_mass_g { 1_000 }
+    standard { 'european_biochar_certificate' }
+  end
+end
diff --git a/spec/integration/estimates_spec.rb b/spec/integration/estimates_spec.rb
index caf2b9c..3e1e2e7 100644
--- a/spec/integration/estimates_spec.rb
+++ b/spec/integration/estimates_spec.rb
@@ -24,15 +24,4 @@
 
     expect(estimates.length).not_to be_zero
   end
-
-  it 'supports create with a project-id' do
-    retrieve_projects_response = Patch::Project.retrieve_projects(page: 1)
-    project_id = retrieve_projects_response.data.first.id
-    create_estimate_response = Patch::Estimate.create_mass_estimate(mass_g: 100, project_id: project_id)
-    estimate_id = create_estimate_response.data.id
-
-    expect(create_estimate_response.success).to eq true
-    expect(create_estimate_response.data.order.id).not_to be_nil
-    expect(create_estimate_response.data.order.mass_g).to eq(100)
-  end
 end
diff --git a/spec/integration/orders_spec.rb b/spec/integration/orders_spec.rb
index 8ea70af..e9bf2d2 100644
--- a/spec/integration/orders_spec.rb
+++ b/spec/integration/orders_spec.rb
@@ -65,7 +65,7 @@
     order = create_order_response.data
 
     expect(order.id).not_to be_nil
-    expect(order.mass_g).to eq(5_000_000)
+    expect(order.mass_g).to eq(5_00_000)
     expect(order.price_cents_usd.to_i).to eq(500)
     expect(order.patch_fee_cents_usd).not_to be_empty
     expect(
diff --git a/spec/models/allocation_spec.rb b/spec/models/allocation_spec.rb
index 52b6467..80cc959 100644
--- a/spec/models/allocation_spec.rb
+++ b/spec/models/allocation_spec.rb
@@ -20,18 +20,25 @@
 describe 'Allocation' do
   before do
     # run before each test
-    @instance = Patch::Allocation.new
+    @instance = build(:allocation)
   end
 
   after do
     # run after each test
   end
 
+  it_behaves_like "a generated class" do
+    let(:instance) { @instance }
+    let(:instance_hash) { { id: @instance.id, mass_g: @instance.mass_g, production: @instance.production } }
+    let(:nullable_properties) { Set.new }
+  end
+
   describe 'test an instance of Allocation' do
     it 'should create an instance of Allocation' do
       expect(@instance).to be_instance_of(Patch::Allocation)
     end
   end
+
   describe 'test attribute "id"' do
     it 'should work' do
       # assertion here. ref: https://www.relishapp.com/rspec/rspec-expectations/docs/built-in-matchers
diff --git a/spec/models/create_mass_estimate_request_spec.rb b/spec/models/create_mass_estimate_request_spec.rb
index d1d6c3a..a001df1 100644
--- a/spec/models/create_mass_estimate_request_spec.rb
+++ b/spec/models/create_mass_estimate_request_spec.rb
@@ -20,13 +20,19 @@
 describe 'CreateMassEstimateRequest' do
   before do
     # run before each test
-    @instance = Patch::CreateMassEstimateRequest.new
+    @instance = build(:create_mass_estimate_request)
   end
 
   after do
     # run after each test
   end
 
+  it_behaves_like "a generated class" do
+    let(:instance) { @instance }
+    let(:instance_hash) { { project_id: @instance.project_id, mass_g: @instance.mass_g } }
+    let(:nullable_properties) { Set.new }
+  end
+
   describe 'test an instance of CreateMassEstimateRequest' do
     it 'should create an instance of CreateMassEstimateRequest' do
       expect(@instance).to be_instance_of(Patch::CreateMassEstimateRequest)
diff --git a/spec/models/create_order_request_spec.rb b/spec/models/create_order_request_spec.rb
index 702f2ac..dd4ed72 100644
--- a/spec/models/create_order_request_spec.rb
+++ b/spec/models/create_order_request_spec.rb
@@ -20,13 +20,19 @@
 describe 'CreateOrderRequest' do
   before do
     # run before each test
-    @instance = Patch::CreateOrderRequest.new
+    @instance = build(:create_order_request)
   end
 
   after do
     # run after each test
   end
 
+  it_behaves_like "a generated class" do
+    let(:instance) { @instance }
+    let(:instance_hash) { { project_id: @instance.project_id, mass_g: @instance.mass_g, total_price_cents_usd: @instance.total_price_cents_usd, metadata: @instance.metadata } }
+    let(:nullable_properties) { Set.new }
+  end
+
   describe 'test an instance of CreateOrderRequest' do
     it 'should create an instance of CreateOrderRequest' do
       expect(@instance).to be_instance_of(Patch::CreateOrderRequest)
diff --git a/spec/models/create_preference_request_spec.rb b/spec/models/create_preference_request_spec.rb
index 4fb6008..619d7b6 100644
--- a/spec/models/create_preference_request_spec.rb
+++ b/spec/models/create_preference_request_spec.rb
@@ -20,18 +20,25 @@
 describe 'CreatePreferenceRequest' do
   before do
     # run before each test
-    @instance = Patch::CreatePreferenceRequest.new
+    @instance = build(:create_preference_request)
   end
 
   after do
     # run after each test
   end
 
+  it_behaves_like "a generated class" do
+    let(:instance) { @instance }
+    let(:instance_hash) { { project_id: @instance.project_id } }
+    let(:nullable_properties) { Set.new }
+  end
+
   describe 'test an instance of CreatePreferenceRequest' do
     it 'should create an instance of CreatePreferenceRequest' do
       expect(@instance).to be_instance_of(Patch::CreatePreferenceRequest)
     end
   end
+
   describe 'test attribute "project_id"' do
     it 'should work' do
       # assertion here. ref: https://www.relishapp.com/rspec/rspec-expectations/docs/built-in-matchers
diff --git a/spec/models/error_response_spec.rb b/spec/models/error_response_spec.rb
index ef7fcf8..ca04fbc 100644
--- a/spec/models/error_response_spec.rb
+++ b/spec/models/error_response_spec.rb
@@ -20,13 +20,19 @@
 describe 'ErrorResponse' do
   before do
     # run before each test
-    @instance = Patch::ErrorResponse.new
+    @instance = build(:error_response)
   end
 
   after do
     # run after each test
   end
 
+  it_behaves_like "a generated class" do
+    let(:instance) { @instance }
+    let(:instance_hash) { { success: @instance.success, error: @instance.error, data: @instance.data } }
+    let(:nullable_properties) { Set.new(["data"]) }
+  end
+
   describe 'test an instance of ErrorResponse' do
     it 'should create an instance of ErrorResponse' do
       expect(@instance).to be_instance_of(Patch::ErrorResponse)
diff --git a/spec/models/estimate_list_response_spec.rb b/spec/models/estimate_list_response_spec.rb
index 35b3220..68c2af6 100644
--- a/spec/models/estimate_list_response_spec.rb
+++ b/spec/models/estimate_list_response_spec.rb
@@ -20,13 +20,19 @@
 describe 'EstimateListResponse' do
   before do
     # run before each test
-    @instance = Patch::EstimateListResponse.new
+    @instance = build(:estimate_list_response)
   end
 
   after do
     # run after each test
   end
 
+  it_behaves_like "a generated class" do
+    let(:instance) { @instance }
+    let(:instance_hash) { { success: @instance.success, error: @instance.error, data: @instance.data, meta: @instance.meta } }
+    let(:nullable_properties) { Set.new(["error"]) }
+  end
+
   describe 'test an instance of EstimateListResponse' do
     it 'should create an instance of EstimateListResponse' do
       expect(@instance).to be_instance_of(Patch::EstimateListResponse)
diff --git a/spec/models/estimate_response_spec.rb b/spec/models/estimate_response_spec.rb
index c206824..1d41216 100644
--- a/spec/models/estimate_response_spec.rb
+++ b/spec/models/estimate_response_spec.rb
@@ -20,13 +20,19 @@
 describe 'EstimateResponse' do
   before do
     # run before each test
-    @instance = Patch::EstimateResponse.new
+    @instance = build(:estimate_response)
   end
 
   after do
     # run after each test
   end
 
+  it_behaves_like "a generated class" do
+    let(:instance) { @instance }
+    let(:instance_hash) { { success: @instance.success, error: @instance.error, data: @instance.data } }
+    let(:nullable_properties) { Set.new(["error"]) }
+  end
+
   describe 'test an instance of EstimateResponse' do
     it 'should create an instance of EstimateResponse' do
       expect(@instance).to be_instance_of(Patch::EstimateResponse)
diff --git a/spec/models/estimate_spec.rb b/spec/models/estimate_spec.rb
index e0dc324..34ddae7 100644
--- a/spec/models/estimate_spec.rb
+++ b/spec/models/estimate_spec.rb
@@ -20,18 +20,25 @@
 describe 'Estimate' do
   before do
     # run before each test
-    @instance = Patch::Estimate.new
+    @instance = build(:estimate)
   end
 
   after do
     # run after each test
   end
 
+  it_behaves_like "a generated class" do
+    let(:instance) { @instance }
+    let(:instance_hash) { { id: @instance.id, order: @instance.order.to_hash, production: @instance.production, type: @instance.type } }
+    let(:nullable_properties) { Set.new(["order"]) }
+  end
+
   describe 'test an instance of Estimate' do
     it 'should create an instance of Estimate' do
       expect(@instance).to be_instance_of(Patch::Estimate)
     end
   end
+
   describe 'test attribute "id"' do
     it 'should work' do
       # assertion here. ref: https://www.relishapp.com/rspec/rspec-expectations/docs/built-in-matchers
diff --git a/spec/models/meta_index_object_spec.rb b/spec/models/meta_index_object_spec.rb
index 43b2db1..0ea5c08 100644
--- a/spec/models/meta_index_object_spec.rb
+++ b/spec/models/meta_index_object_spec.rb
@@ -20,13 +20,19 @@
 describe 'MetaIndexObject' do
   before do
     # run before each test
-    @instance = Patch::MetaIndexObject.new
+    @instance = build(:meta_index_object)
   end
 
   after do
     # run after each test
   end
 
+  it_behaves_like "a generated class" do
+    let(:instance) { @instance }
+    let(:instance_hash) { { next_page: @instance.next_page, prev_page: @instance.prev_page } }
+    let(:nullable_properties) { Set.new(["next_page", "prev_page"]) }
+  end
+
   describe 'test an instance of MetaIndexObject' do
     it 'should create an instance of MetaIndexObject' do
       expect(@instance).to be_instance_of(Patch::MetaIndexObject)
diff --git a/spec/models/order_list_response_spec.rb b/spec/models/order_list_response_spec.rb
index db4238a..4e85078 100644
--- a/spec/models/order_list_response_spec.rb
+++ b/spec/models/order_list_response_spec.rb
@@ -20,13 +20,19 @@
 describe 'OrderListResponse' do
   before do
     # run before each test
-    @instance = Patch::OrderListResponse.new
+    @instance = build(:order_list_response)
   end
 
   after do
     # run after each test
   end
 
+  it_behaves_like "a generated class" do
+    let(:instance) { @instance }
+    let(:instance_hash) { { success: @instance.success, error: @instance.error, data: @instance.data, meta: @instance.meta } }
+    let(:nullable_properties) { Set.new(["error"]) }
+  end
+
   describe 'test an instance of OrderListResponse' do
     it 'should create an instance of OrderListResponse' do
       expect(@instance).to be_instance_of(Patch::OrderListResponse)
diff --git a/spec/models/order_response_spec.rb b/spec/models/order_response_spec.rb
index dd2758a..9880ff9 100644
--- a/spec/models/order_response_spec.rb
+++ b/spec/models/order_response_spec.rb
@@ -20,13 +20,19 @@
 describe 'OrderResponse' do
   before do
     # run before each test
-    @instance = Patch::OrderResponse.new
+    @instance = build(:order_response)
   end
 
   after do
     # run after each test
   end
 
+  it_behaves_like "a generated class" do
+    let(:instance) { @instance }
+    let(:instance_hash) { { success: @instance.success, error: @instance.error, data: @instance.data } }
+    let(:nullable_properties) { Set.new(["error"]) }
+  end
+
   describe 'test an instance of OrderResponse' do
     it 'should create an instance of OrderResponse' do
       expect(@instance).to be_instance_of(Patch::OrderResponse)
diff --git a/spec/models/order_spec.rb b/spec/models/order_spec.rb
index 1a79a7c..a1e49d7 100644
--- a/spec/models/order_spec.rb
+++ b/spec/models/order_spec.rb
@@ -20,13 +20,30 @@
 describe 'Order' do
   before do
     # run before each test
-    @instance = Patch::Order.new
+    @instance = build(:order)
   end
 
   after do
     # run after each test
   end
 
+  it_behaves_like "a generated class" do
+    let(:instance) { @instance }
+    let(:instance_hash) {
+      {
+        id: @instance.id,
+        mass_g: @instance.mass_g,
+        production: @instance.production,
+        state: @instance.state,
+        allocation_state: @instance.allocation_state,
+        price_cents_usd: @instance.price_cents_usd,
+        patch_fee_cents_usd: @instance.patch_fee_cents_usd,
+        metadata: @instance.metadata
+      }
+    }
+    let(:nullable_properties) { Set.new(["patch_fee_cents_usd", "price_cents_usd"]) }
+  end
+
   describe 'test an instance of Order' do
     it 'should create an instance of Order' do
       expect(@instance).to be_instance_of(Patch::Order)
diff --git a/spec/models/preference_list_response_spec.rb b/spec/models/preference_list_response_spec.rb
index d85a79d..7388d7f 100644
--- a/spec/models/preference_list_response_spec.rb
+++ b/spec/models/preference_list_response_spec.rb
@@ -20,13 +20,19 @@
 describe 'PreferenceListResponse' do
   before do
     # run before each test
-    @instance = Patch::PreferenceListResponse.new
+    @instance = build(:preference_list_response)
   end
 
   after do
     # run after each test
   end
 
+  it_behaves_like "a generated class" do
+    let(:instance) { @instance }
+    let(:instance_hash) { { success: @instance.success, error: @instance.error, data: @instance.data, meta: @instance.meta } }
+    let(:nullable_properties) { Set.new(["error"]) }
+  end
+
   describe 'test an instance of PreferenceListResponse' do
     it 'should create an instance of PreferenceListResponse' do
       expect(@instance).to be_instance_of(Patch::PreferenceListResponse)
diff --git a/spec/models/preference_response_spec.rb b/spec/models/preference_response_spec.rb
index a171d1e..e19ad9d 100644
--- a/spec/models/preference_response_spec.rb
+++ b/spec/models/preference_response_spec.rb
@@ -20,13 +20,19 @@
 describe 'PreferenceResponse' do
   before do
     # run before each test
-    @instance = Patch::PreferenceResponse.new
+    @instance = build(:preference_response)
   end
 
   after do
     # run after each test
   end
 
+  it_behaves_like "a generated class" do
+    let(:instance) { @instance }
+    let(:instance_hash) { { success: @instance.success, error: @instance.error, data: @instance.data } }
+    let(:nullable_properties) { Set.new(["error"]) }
+  end
+
   describe 'test an instance of PreferenceResponse' do
     it 'should create an instance of PreferenceResponse' do
       expect(@instance).to be_instance_of(Patch::PreferenceResponse)
diff --git a/spec/models/preference_spec.rb b/spec/models/preference_spec.rb
index 54e21ca..abf0fe9 100644
--- a/spec/models/preference_spec.rb
+++ b/spec/models/preference_spec.rb
@@ -20,13 +20,19 @@
 describe 'Preference' do
   before do
     # run before each test
-    @instance = Patch::Preference.new
+    @instance = build(:preference)
   end
 
   after do
     # run after each test
   end
 
+  it_behaves_like "a generated class" do
+    let(:instance) { @instance }
+    let(:instance_hash) { { id: @instance.id, project: @instance.project.to_hash, allocation_percentage: @instance.allocation_percentage } }
+    let(:nullable_properties) { Set.new }
+  end
+
   describe 'test an instance of Preference' do
     it 'should create an instance of Preference' do
       expect(@instance).to be_instance_of(Patch::Preference)
diff --git a/spec/models/project_list_response_spec.rb b/spec/models/project_list_response_spec.rb
index a2897f3..c83f631 100644
--- a/spec/models/project_list_response_spec.rb
+++ b/spec/models/project_list_response_spec.rb
@@ -20,13 +20,19 @@
 describe 'ProjectListResponse' do
   before do
     # run before each test
-    @instance = Patch::ProjectListResponse.new
+    @instance = build(:project_list_response)
   end
 
   after do
     # run after each test
   end
 
+  it_behaves_like "a generated class" do
+    let(:instance) { @instance }
+    let(:instance_hash) { { success: @instance.success, error: @instance.error, data: @instance.data, meta: @instance.meta } }
+    let(:nullable_properties) { Set.new(["error"]) }
+  end
+
   describe 'test an instance of ProjectListResponse' do
     it 'should create an instance of ProjectListResponse' do
       expect(@instance).to be_instance_of(Patch::ProjectListResponse)
diff --git a/spec/models/project_response_spec.rb b/spec/models/project_response_spec.rb
index 3bfee28..49f0965 100644
--- a/spec/models/project_response_spec.rb
+++ b/spec/models/project_response_spec.rb
@@ -20,13 +20,19 @@
 describe 'ProjectResponse' do
   before do
     # run before each test
-    @instance = Patch::ProjectResponse.new
+    @instance = build(:project_response)
   end
 
   after do
     # run after each test
   end
 
+  it_behaves_like "a generated class" do
+    let(:instance) { @instance }
+    let(:instance_hash) { { success: @instance.success, error: @instance.error, data: @instance.data } }
+    let(:nullable_properties) { Set.new(["error"]) }
+  end
+
   describe 'test an instance of ProjectResponse' do
     it 'should create an instance of ProjectResponse' do
       expect(@instance).to be_instance_of(Patch::ProjectResponse)
diff --git a/spec/models/project_spec.rb b/spec/models/project_spec.rb
index 42e509f..bc13bfa 100644
--- a/spec/models/project_spec.rb
+++ b/spec/models/project_spec.rb
@@ -20,13 +20,33 @@
 describe 'Project' do
   before do
     # run before each test
-    @instance = Patch::Project.new
+    @instance = build(:project)
   end
 
   after do
     # run after each test
   end
 
+  it_behaves_like "a generated class" do
+    let(:instance) { @instance }
+    let(:instance_hash) {
+      {
+        id: @instance.id,
+        name: @instance.name,
+        description: @instance.description,
+        production: @instance.production,
+        type: @instance.type,
+        country: @instance.country,
+        developer: @instance.developer,
+        photos: @instance.photos,
+        average_price_per_tonne_cents_usd: @instance.average_price_per_tonne_cents_usd,
+        remaining_mass_g: @instance.remaining_mass_g,
+        standard: @instance.standard
+      }
+    }
+    let(:nullable_properties) { Set.new(["photos", "standard"]) }
+  end
+
   describe 'test an instance of Project' do
     it 'should create an instance of Project' do
       expect(@instance).to be_instance_of(Patch::Project)
diff --git a/spec/spec_helper.rb b/spec/spec_helper.rb
index 2ed41b9..f9bc511 100644
--- a/spec/spec_helper.rb
+++ b/spec/spec_helper.rb
@@ -13,6 +13,10 @@
 # load the gem
 require 'patch_ruby'
 require 'constants'
+require 'factory_bot'
+
+# Require shared spec examples
+Dir["./spec/support/**/*.rb"].sort.each { |f| require f }
 
 # The following  was generated by the `rspec --init` command. Conventionally, all
 # specs live under a `spec` directory, which RSpec adds to the `$LOAD_PATH`.
@@ -33,6 +37,12 @@
 #
 # See http://rubydoc.info/gems/rspec-core/RSpec/Core/Configuration
 RSpec.configure do |config|
+  # Use FactoryBot for test factories
+  config.include FactoryBot::Syntax::Methods
+  config.before(:suite) do
+    FactoryBot.find_definitions
+  end
+
   # rspec-expectations config goes here. You can use an alternate
   # assertion/expectation library such as wrong or the stdlib/minitest
   # assertions if you prefer.
diff --git a/spec/support/shared/generated_classes.rb b/spec/support/shared/generated_classes.rb
new file mode 100644
index 0000000..b9cf3d5
--- /dev/null
+++ b/spec/support/shared/generated_classes.rb
@@ -0,0 +1,13 @@
+RSpec.shared_examples "a generated class" do
+  describe '.openapi_nullable' do
+    it 'returns a set with nullable properties' do
+      expect(instance.class.openapi_nullable).to eq nullable_properties
+    end
+  end
+
+  describe '#to_hash' do
+    it 'returns a set with nullable properties' do
+      expect(@instance.to_hash).to eq(instance_hash)
+    end
+  end
+end