diff --git a/app/controllers/donations_controller.rb b/app/controllers/donations_controller.rb
index bc9e6c7e06..2d67085062 100644
--- a/app/controllers/donations_controller.rb
+++ b/app/controllers/donations_controller.rb
@@ -97,8 +97,9 @@ def show
 
   def update
     @donation = Donation.find(params[:id])
+    previous_quantities = @donation.line_items_quantities
     if @donation.update_attributes(donation_params)
-      @donation.storage_location.adjust_from_past! @donation
+      @donation.storage_location.adjust_from_past!(@donation, previous_quantities)
     redirect_to donations_path
     else
       render 'edit'
diff --git a/app/controllers/purchases_controller.rb b/app/controllers/purchases_controller.rb
index 7f4b4cb69d..ec28a6dba6 100644
--- a/app/controllers/purchases_controller.rb
+++ b/app/controllers/purchases_controller.rb
@@ -3,7 +3,7 @@ class PurchasesController < ApplicationController
   skip_before_action :verify_authenticity_token, only: [:scale_intake, :scale]
   skip_before_action :authenticate_user!, only: [:scale_intake, :scale]
   skip_before_action :authorize_user, only: [:scale_intake, :scale]
-  
+
   def index
     @purchases = current_organization.purchases
                      .includes(:line_items, :storage_location)
@@ -49,8 +49,9 @@ def show
   def update
     @purchase = current_organization.purchases.find(params[:id])
     @purchase.changed?
+    previous_quantities = @purchase.line_items_quantities
    if @purchase.update_attributes(purchase_params)
-     @purchase.storage_location.adjust_from_past! @purchase
+     @purchase.storage_location.adjust_from_past!(@purchase, previous_quantities)
     redirect_to purchases_path
    else
      render 'edit'
diff --git a/app/models/concerns/itemizable.rb b/app/models/concerns/itemizable.rb
index d0343c3184..5c99a490b6 100644
--- a/app/models/concerns/itemizable.rb
+++ b/app/models/concerns/itemizable.rb
@@ -62,6 +62,13 @@ def total
     validates_associated :line_items
   end
 
+  def line_items_quantities
+    line_items.inject(Hash.new) do |hash, item|
+      hash[item.id] = item.quantity
+      hash
+    end
+  end
+
   private
 
   def line_item_items_exist_in_inventory
diff --git a/app/models/storage_location.rb b/app/models/storage_location.rb
index 66e6eb7792..672ccbd5cb 100644
--- a/app/models/storage_location.rb
+++ b/app/models/storage_location.rb
@@ -101,24 +101,16 @@ def remove!(donation_or_purchase)
     log
   end
 
-  def adjust_from_past!(donation_or_purchase)
-    log = {}
+  def adjust_from_past!(donation_or_purchase, previous_quantities)
     donation_or_purchase.line_items.each do |line_item|
-      inventory_item = InventoryItem.find_or_create_by(storage_location_id: self.id,
-                                                       item_id: line_item.item_id)
-      before_last_save = line_item.quantity.nil? ? 0 : line_item.quantity
-      updated_quantity = inventory_item.quantity.nil? ? 0 : inventory_item.quantity
-      delta = before_last_save - updated_quantity
-      inventory_item.quantity += delta rescue 0
-      if inventory_item.quantity <= 0
-        inventory_item.destroy
-      else
-        # Otherwise update it        
-        inventory_item.save
+      inventory_item = InventoryItem.find_or_create_by(storage_location_id: self.id, item_id: line_item.item_id)
+      if previous_quantity = previous_quantities[line_item.id]
+        inventory_item.quantity += line_item.quantity
+        inventory_item.quantity -= previous_quantity
+        inventory_item.save!
       end
-      log[line_item.item_id] = "+#{line_item.quantity}"
+      inventory_item.destroy! if inventory_item.quantity == 0
     end
-    log
   end
 
   def distribute!(distribution)
diff --git a/spec/controllers/donations_controller_spec.rb b/spec/controllers/donations_controller_spec.rb
index 03b5f336fa..7ca69279c2 100644
--- a/spec/controllers/donations_controller_spec.rb
+++ b/spec/controllers/donations_controller_spec.rb
@@ -52,6 +52,23 @@
         put :update, params: default_params.merge(id: donation.id, donation: { source: "Donation Site" })
         expect(response).to redirect_to(donations_path)
       end
+
+      it "updates storage quantity correctly" do
+        donation = create(:donation, :with_items, item_quantity: 10)
+        line_item = donation.line_items.first
+        line_item_params = {
+          "0" => {
+            "_destroy" => "false",
+            item_id: line_item.item_id,
+            quantity: "15",
+            id: line_item.id
+          }
+        }
+        donation_params = { source: "Donation Site", line_items_attributes: line_item_params }
+        expect {
+          put :update, params: default_params.merge(id: donation.id, donation: donation_params)
+        }.to change { donation.storage_location.inventory_items.first.quantity }.by(5)
+      end
     end
 
     describe "GET #edit" do
diff --git a/spec/controllers/purchases_controller_spec.rb b/spec/controllers/purchases_controller_spec.rb
index 56699e5b08..4898ea3dd7 100644
--- a/spec/controllers/purchases_controller_spec.rb
+++ b/spec/controllers/purchases_controller_spec.rb
@@ -1,7 +1,7 @@
 require 'rails_helper'
 
 RSpec.describe PurchasesController, type: :controller do
-  
+
   let(:default_params) {
     { organization_id: @organization.to_param }
   }
@@ -53,6 +53,23 @@
         put :update, params: default_params.merge(id: purchase.id, purchase: { purchased_from: "Google" })
         expect(response).to redirect_to(purchases_path)
       end
+
+      it "updates storage quantity correctly" do
+        purchase = create(:purchase, :with_items, item_quantity: 10)
+        line_item = purchase.line_items.first
+        line_item_params = {
+          "0" => {
+            "_destroy" => "false",
+            item_id: line_item.item_id,
+            quantity: "5",
+            id: line_item.id
+          }
+        }
+        purchase_params = { source: "Purchase Site", line_items_attributes: line_item_params }
+        expect {
+          put :update, params: default_params.merge(id: purchase.id, purchase: purchase_params)
+        }.to change { purchase.storage_location.inventory_items.first.quantity }.by(-5)
+      end
     end
 
     describe "GET #edit" do
diff --git a/spec/models/storage_location_spec.rb b/spec/models/storage_location_spec.rb
index bd859f490c..42bb573764 100644
--- a/spec/models/storage_location_spec.rb
+++ b/spec/models/storage_location_spec.rb
@@ -127,28 +127,20 @@
         end
 
         it "updates the quantity of items" do
+          previous_quantities = donation.line_items_quantities
           donation.line_items.first.update(quantity: 5)
           expect {
-            storage_location.adjust_from_past!(donation)
+            storage_location.adjust_from_past!(donation, previous_quantities)
             storage_location.reload
           }.to change{storage_location.size}.by(-5)
         end
 
-
-        it "adds an inventory item if it doesn't already exist" do
-          donation.line_items << create(:line_item, quantity: 5)
-          expect {
-            storage_location.adjust_from_past!(donation)
-            storage_location.reload
-          }.to change{storage_location.size}.by(5)
-          .and change{InventoryItem.count}.by(1)
-        end
-
         it "removes the inventory item from the DB if the item's removal results in a 0 count" do
+          previous_quantities = donation.line_items_quantities
           donation.line_items.first.update(quantity: 0)
 
           expect {
-            storage_location.adjust_from_past!(donation)
+            storage_location.adjust_from_past!(donation, previous_quantities)
             storage_location.reload
           }.to change{storage_location.inventory_items.size}.by(-1)
           .and change{InventoryItem.count}.by(-1)
@@ -159,27 +151,13 @@
           storage_location.intake!(purchase)
           storage_location.items.reload
         end
-        it "add additional line item" do
-          item = create(:item)
-          purchase.line_items.create(item_id: item.id, quantity: 6)
-          storage_location.adjust_from_past!(purchase)
-          storage_location.items.reload
-        end
-        it "adds an inventory item if it doesn't already exist" do
-          purchase.line_items << create(:line_item, quantity: 5)
-
-          expect {
-            storage_location.adjust_from_past!(purchase)
-            storage_location.reload
-          }.to change{storage_location.size}.by(5)
-          .and change{InventoryItem.count}.by(1)
-        end
 
         it "removes the inventory item from the DB if the item's removal results in a 0 count" do
+          previous_quantities = purchase.line_items_quantities
           purchase.line_items.first.update(quantity: 0)
 
           expect {
-            storage_location.adjust_from_past!(purchase)
+            storage_location.adjust_from_past!(purchase, previous_quantities)
             storage_location.reload
           }.to change{storage_location.inventory_items.size}.by(-1)
           .and change{InventoryItem.count}.by(-1)
diff --git a/spec/support/itemizable_shared_example.rb b/spec/support/itemizable_shared_example.rb
index 9fcb0471ec..0f7c525500 100644
--- a/spec/support/itemizable_shared_example.rb
+++ b/spec/support/itemizable_shared_example.rb
@@ -115,5 +115,20 @@
         }.to change{ subject.line_items.total }.by(10)
       end
     end
+
+    describe "updated line item quantity" do
+      subject { create(model_f, organization: @organization) }
+      let(:storage_location) { subject.storage_location }
+
+      it "updates storage location quantity by the correct amount" do
+        line_item = subject.line_items.create(item_id: item.id, quantity: 10)
+        inventory_item = create(:inventory_item, storage_location: storage_location, item_id: line_item.item_id)
+        previous_quantities = subject.line_items_quantities
+        line_item.update!(quantity: 5)
+        expect{
+          storage_location.adjust_from_past!(subject, previous_quantities)
+        }.to change{ inventory_item.reload.quantity }.by(-5)
+      end
+    end
   end
 end