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

activerecord/lib/active_record/associations/builder/has_and_belongs_to_many.rb

@@ -15,7 +15,10 @@ def join_table
         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 @@ def self.build(lhs_class, name, options)
           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

activerecord/test/cases/associations/has_and_belongs_to_many_associations_test.rb

@@ -869,6 +869,15 @@ def test_namespaced_habtm
     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")

activerecord/test/models/publisher/article.rb

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

activerecord/test/schema/schema.rb

@@ -62,6 +62,11 @@ def except(adapter_names_to_exclude)
     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