thrown ActiveRecord::AssociationTypeMismatch when assigning a wrong value for a namespaced association

fixes #20541

3 files changed, 31 insertions, 3 deletions

diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index 3351013b16..0ec3902eac 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,10 @@
+*   Correctly thrown `ActiveRecord::AssociationTypeMismatch` when assigning
+    a wrong value to a namespaced association.
+
+    Fixes #20545
+
+    *Diego Carrion*
+
 *   `validates_absence_of` respects `marked_for_destruction?`.
 
     Fixes #20449.
diff --git a/activerecord/lib/active_record/associations/association.rb b/activerecord/lib/active_record/associations/association.rb
index 930f678ae8..7c729676a7 100644
--- a/activerecord/lib/active_record/associations/association.rb
+++ b/activerecord/lib/active_record/associations/association.rb
@@ -211,9 +211,12 @@ module ActiveRecord
         # the kind of the class of the associated objects. Meant to be used as
         # a sanity check when you are about to assign an associated record.
         def raise_on_type_mismatch!(record)
-          unless record.is_a?(reflection.klass) || record.is_a?(reflection.class_name.constantize)
-            message = "#{reflection.class_name}(##{reflection.klass.object_id}) expected, got #{record.class}(##{record.class.object_id})"
-            raise ActiveRecord::AssociationTypeMismatch, message
+          unless record.is_a?(reflection.klass)
+            fresh_class = reflection.class_name.safe_constantize
+            unless fresh_class && record.is_a?(fresh_class)
+              message = "#{reflection.class_name}(##{reflection.klass.object_id}) expected, got #{record.class}(##{record.class.object_id})"
+              raise ActiveRecord::AssociationTypeMismatch, message
+            end
           end
         end
 
diff --git a/activerecord/test/cases/associations/belongs_to_associations_test.rb b/activerecord/test/cases/associations/belongs_to_associations_test.rb
index 039cc46b0b..7726829be4 100644
--- a/activerecord/test/cases/associations/belongs_to_associations_test.rb
+++ b/activerecord/test/cases/associations/belongs_to_associations_test.rb
@@ -19,6 +19,8 @@ require 'models/invoice'
 require 'models/line_item'
 require 'models/column'
 require 'models/record'
+require 'models/admin'
+require 'models/admin/user'
 
 class BelongsToAssociationsTest < ActiveRecord::TestCase
   fixtures :accounts, :companies, :developers, :projects, :topics,
@@ -151,6 +153,22 @@ class BelongsToAssociationsTest < ActiveRecord::TestCase
     assert_raise(ActiveRecord::AssociationTypeMismatch) { Account.find(1).firm = Project.find(1) }
   end
 
+  def test_type_mismatch_with_namespaced_class
+    assert_nil defined?(Region), "This test should be done with a only namespaced class"
+    ActiveRecord::Base.connection.create_table(:admin_regions) { |t| t.string :name }
+    ActiveRecord::Base.connection.add_column :admin_users, :region_id, :integer
+    Admin.const_set "RegionalUser", Class.new(Admin::User) { belongs_to(:region) }
+    Admin.const_set "Region", Class.new(ActiveRecord::Base)
+    e = assert_raise(ActiveRecord::AssociationTypeMismatch) { Admin::RegionalUser.new(region: 'wrong value') }
+    assert_match(/^Region\([^)]+\) expected, got String\([^)]+\)$/, e.message)
+  ensure
+    Admin.send :remove_const, "Region" if Admin.const_defined?("Region")
+    Admin.send :remove_const, "RegionalUser" if Admin.const_defined?("RegionalUser")
+    connection = ActiveRecord::Base.connection
+    connection.remove_column :admin_users, :region_id if connection.column_exists?(:admin_users, :region_id)
+    connection.drop_table :admin_regions if connection.table_exists?(:admin_regions)
+  end
+
   def test_natural_assignment
     apple = Firm.create("name" => "Apple")
     citibank = Account.create("credit_limit" => 10)