Fix has_and_belongs_to_many in a namespaced model pointing to a non namespaced model

Now the following case will work fine

    class Tag < ActiveRecord::Base
    end

    class Publisher::Article < ActiveRecord::Base
      has_and_belongs_to_many :tags
    end

Fixes #15761

4 files changed, 20 insertions, 8 deletions

diff --git a/activerecord/lib/active_record/associations/builder/has_and_belongs_to_many.rb b/activerecord/lib/active_record/associations/builder/has_and_belongs_to_many.rb
index 0ad5206980..34a555dfd4 100644
--- a/activerecord/lib/active_record/associations/builder/has_and_belongs_to_many.rb
+++ b/activerecord/lib/active_record/associations/builder/has_and_belongs_to_many.rb
@@ -15,7 +15,10 @@ module ActiveRecord::Associations::Builder
         end
 
         private
-        def klass; @rhs_class_name.constantize; end
+
+        def klass
+          @lhs_class.send(:compute_type, @rhs_class_name)
+        end
       end
 
       def self.build(lhs_class, name, options)
@@ -23,13 +26,7 @@ module ActiveRecord::Associations::Builder
           KnownTable.new options[:join_table].to_s
         else
           class_name = options.fetch(:class_name) {
-            model_name = name.to_s.camelize.singularize
-
-            if lhs_class.parent_name
-              model_name.prepend("#{lhs_class.parent_name}::")
-            end
-
-            model_name
+            name.to_s.camelize.singularize
           }
           KnownClass.new lhs_class, class_name
         end
diff --git a/activerecord/test/cases/associations/has_and_belongs_to_many_associations_test.rb b/activerecord/test/cases/associations/has_and_belongs_to_many_associations_test.rb
index 080c499444..3f8b26980f 100644
--- a/activerecord/test/cases/associations/has_and_belongs_to_many_associations_test.rb
+++ b/activerecord/test/cases/associations/has_and_belongs_to_many_associations_test.rb
@@ -869,6 +869,15 @@ class HasAndBelongsToManyAssociationsTest < ActiveRecord::TestCase
     assert_includes magazine.articles, article
   end
 
+  def test_has_and_belongs_to_many_in_a_namespaced_model_pointing_to_a_non_namespaced_model
+    article = Publisher::Article.create
+    tag = Tag.create
+    article.tags << tag
+    article.save
+
+    assert_includes article.tags, tag
+  end
+
   def test_redefine_habtm
     child = SubDeveloper.new("name" => "Aredridel")
     child.special_projects << SpecialProject.new("name" => "Special Project")
diff --git a/activerecord/test/models/publisher/article.rb b/activerecord/test/models/publisher/article.rb
index 03a277bbdd..d73a8eb936 100644
--- a/activerecord/test/models/publisher/article.rb
+++ b/activerecord/test/models/publisher/article.rb
@@ -1,3 +1,4 @@
 class Publisher::Article < ActiveRecord::Base
   has_and_belongs_to_many :magazines
+  has_and_belongs_to_many :tags
 end
diff --git a/activerecord/test/schema/schema.rb b/activerecord/test/schema/schema.rb
index a7c7fc70bc..fd85050dd4 100644
--- a/activerecord/test/schema/schema.rb
+++ b/activerecord/test/schema/schema.rb
@@ -62,6 +62,11 @@ ActiveRecord::Schema.define do
     t.references :magazine
   end
 
+  create_table :articles_tags, force: true do |t|
+    t.references :article
+    t.references :tag
+  end
+
   create_table :audit_logs, force: true do |t|
     t.column :message, :string, null: false
     t.column :developer_id, :integer, null: false