diff --git a/activerecord/lib/active_record/associations.rb b/activerecord/lib/active_record/associations.rb
index 0809b27..7135e50 100644
--- a/activerecord/lib/active_record/associations.rb
+++ b/activerecord/lib/active_record/associations.rb
@@ -1094,6 +1094,13 @@ module ActiveRecord
             instance_variable_set(ivar, new_value.nil? ? nil : association)
           end
 
+          if association_proxy_class == BelongsToAssociation
+            define_method("#{reflection.primary_key_name}=") do |target_id|
+              instance_variable_set(ivar, nil)
+              self["#{reflection.primary_key_name}"] = target_id
+            end
+          end
+
           define_method("set_#{reflection.name}_target") do |target|
             return if target.nil? and association_proxy_class == BelongsToAssociation
             association = association_proxy_class.new(self, reflection)
diff --git a/activerecord/test/cases/associations/belongs_to_associations_test.rb b/activerecord/test/cases/associations/belongs_to_associations_test.rb
index b8ec911..c130db2 100644
--- a/activerecord/test/cases/associations/belongs_to_associations_test.rb
+++ b/activerecord/test/cases/associations/belongs_to_associations_test.rb
@@ -45,6 +45,13 @@ class BelongsToAssociationsTest < ActiveRecord::TestCase
     assert_equal apple.id, citibank.firm_id
   end
 
+  def test_foreign_key_assignment
+    signals37 = accounts(:signals37)
+    assert_equal companies(:first_firm), signals37.firm
+    signals37.firm_id = companies(:another_firm).id
+    assert_equal companies(:another_firm), signals37.firm
+  end
+
   def test_no_unexpected_aliasing
     first_firm = companies(:first_firm)
     another_firm = companies(:another_firm)