Add Relation#pick as short-hand for single-value plucks (rails#31941)

* Add Relation#pick as short-hand for single-value plucks

Author: David Heinemeier Hansson

activerecord/lib/active_record/relation/calculations.rb

@@ -200,6 +200,24 @@ def pluck(*column_names)
       end
     end
 
+    # Pick the value(s) from the named column(s) in the current relation.
+    # This is short-hand for `relation.limit(1).pluck(*column_names).first`, and is primarily useful
+    # when you have a relation that's already narrowed down to a single row.
+    #
+    # Just like #pluck, #pick will only load the actual value, not the entire record object, so it's also
+    # more efficient. The value is, again like with pluck, typecast by the column type.
+    #
+    #   Person.where(id: 1).pick(:name)
+    #   # SELECT people.name FROM people WHERE id = 1 LIMIT 1
+    #   # => 'David'
+    #
+    #   Person.where(id: 1).pick(:name, :email_address)
+    #   # SELECT people.name, people.email_address FROM people WHERE id = 1 LIMIT 1
+    #   # => [ 'David', 'david@loudthinking.com' ]
+    def pick(*column_names)
+      limit(1).pluck(*column_names).first
+    end
+
     # Pluck all the ID's for the relation using the table's primary key
     #
     #   Person.ids # SELECT people.id FROM people

activerecord/test/cases/calculations_test.rb

@@ -793,6 +793,16 @@ def test_pluck_loaded_relation_sql_fragment
     end
   end
 
+  def test_pick_one
+    assert_equal "The First Topic", Topic.order(:id).pick(:heading)
+    assert_nil Topic.where("1=0").pick(:heading)
+  end
+
+  def test_pick_two
+    assert_equal ["David", "david@loudthinking.com"], Topic.order(:id).pick(:author_name, :author_email_address)
+    assert_nil Topic.where("1=0").pick(:author_name, :author_email_address)
+  end
+
   def test_grouped_calculation_with_polymorphic_relation
     part = ShipPart.create!(name: "has trinket")
     part.trinkets.create!