Setting the id of a belongs_to object updates all referenced objects [#2989 state:resolved]

activerecord/lib/active_record/associations.rb

@@ -1223,12 +1223,14 @@ def belongs_to(association_id, options = {})
 
         if reflection.options[:polymorphic]
           association_accessor_methods(reflection, BelongsToPolymorphicAssociation)
+          association_foreign_type_setter_method(reflection)
         else
           association_accessor_methods(reflection, BelongsToAssociation)
           association_constructor_method(:build,  reflection, BelongsToAssociation)
           association_constructor_method(:create, reflection, BelongsToAssociation)
         end
 
+        association_foreign_key_setter_method(reflection)
         add_counter_cache_callbacks(reflection)          if options[:counter_cache]
         add_touch_callbacks(reflection, options[:touch]) if options[:touch]
 
@@ -1555,6 +1557,41 @@ def association_constructor_method(constructor, reflection, association_proxy_cl
           end
         end
 
+        def association_foreign_key_setter_method(reflection)
+          setters = reflect_on_all_associations(:belongs_to).map do |belongs_to_reflection|
+            if belongs_to_reflection.primary_key_name == reflection.primary_key_name
+              "association_instance_set(:#{belongs_to_reflection.name}, nil);"
+            end
+          end.compact.join
+
+          class_eval <<-FILE, __FILE__, __LINE__ + 1
+            def #{reflection.primary_key_name}=(new_id)
+              write_attribute :#{reflection.primary_key_name}, new_id
+              if #{reflection.primary_key_name}_changed?
+                #{ setters }
+              end
+            end
+          FILE
+        end
+
+        def association_foreign_type_setter_method(reflection)
+          setters = reflect_on_all_associations(:belongs_to).map do |belongs_to_reflection|
+            if belongs_to_reflection.options[:foreign_type] == reflection.options[:foreign_type]
+              "association_instance_set(:#{belongs_to_reflection.name}, nil);"
+            end
+          end.compact.join
+
+          field = reflection.options[:foreign_type]
+          class_eval <<-FILE, __FILE__, __LINE__ + 1
+            def #{field}=(new_id)
+              write_attribute :#{field}, new_id
+              if #{field}_changed?
+                #{ setters }
+              end
+            end
+          FILE
+        end
+
         def add_counter_cache_callbacks(reflection)
           cache_column = reflection.counter_cache_column
 

activerecord/test/cases/associations/belongs_to_associations_test.rb

@@ -486,4 +486,41 @@ def test_attributes_are_being_set_when_initialized_from_belongs_to_association_w
     new_firm = accounts(:signals37).build_firm(:name => 'Apple')
     assert_equal new_firm.name, "Apple"
   end
+
+  def test_reassigning_the_parent_id_updates_the_object
+    original_parent = Firm.create! :name => "original"
+    updated_parent = Firm.create! :name => "updated"
+
+    client = Client.new("client_of" => original_parent.id)
+    assert_equal original_parent, client.firm
+    assert_equal original_parent, client.firm_with_condition
+    assert_equal original_parent, client.firm_with_other_name
+
+    client.client_of = updated_parent.id
+    assert_equal updated_parent, client.firm
+    assert_equal updated_parent, client.firm_with_condition
+    assert_equal updated_parent, client.firm_with_other_name
+  end
+
+  def test_polymorphic_reassignment_of_associated_id_updates_the_object
+    member1 = Member.create!
+    member2 = Member.create!
+
+    sponsor = Sponsor.new("sponsorable_type" => "Member", "sponsorable_id" => member1.id)
+    assert_equal member1, sponsor.sponsorable
+
+    sponsor.sponsorable_id = member2.id
+    assert_equal member2, sponsor.sponsorable
+  end
+
+  def test_polymorphic_reassignment_of_associated_type_updates_the_object
+    member1 = Member.create!
+
+    sponsor = Sponsor.new("sponsorable_type" => "Member", "sponsorable_id" => member1.id)
+    assert_equal member1, sponsor.sponsorable
+
+    sponsor.sponsorable_type = "Firm"
+    assert_not_equal member1, sponsor.sponsorable
+  end
+
 end