Fix how to compute class name on habtm namespaced.

Thank's for @laurocaetano for the help with tests. 😃

Fixes #14709

activerecord/CHANGELOG.md

+*   Fix how to calculate associated class name when using `habtm` namespaced.
+
+    Fixes #14709.
+
+    *Kassio Borges*
+
 *   `change_column_default` allows `[]` as argument to `change_column_default`.
 
     Fixes #11586.

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

@@ -23,7 +23,11 @@ def self.build(lhs_class, name, options)
           KnownTable.new options[:join_table].to_s
         else
           class_name = options.fetch(:class_name) {
-            name.to_s.camelize.singularize
+            model_name = name.to_s.camelize.singularize
+            if parent_name = lhs_class.parent_name.presence
+              model_name = model_name.prepend("#{parent_name}::")
+            end
+            model_name
           }
           KnownClass.new lhs_class, class_name
         end

activerecord/test/cases/associations/has_and_belongs_to_many_associations_test.rb

@@ -22,6 +22,9 @@
 require 'models/country'
 require 'models/treaty'
 require 'models/vertex'
+require 'models/publisher'
+require 'models/publisher/article'
+require 'models/publisher/magazine'
 require 'active_support/core_ext/string/conversions'
 
 class ProjectWithAfterCreateHook < ActiveRecord::Base
@@ -848,4 +851,13 @@ def test_association_with_validate_false_does_not_run_associated_validation_call
   def test_custom_join_table
     assert_equal 'edges', Vertex.reflect_on_association(:sources).join_table
   end
+
+  def test_namespaced_habtm
+    magazine = Publisher::Magazine.create
+    article = Publisher::Article.create
+    magazine.articles << article
+    magazine.save
+
+    assert_includes magazine.articles, article
+  end
 end

activerecord/test/models/publisher.rb

+module Publisher
+end

activerecord/test/models/publisher/article.rb

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

activerecord/test/models/publisher/magazine.rb

+class Publisher::Magazine < ActiveRecord::Base
+  has_and_belongs_to_many :articles
+end

activerecord/test/schema/schema.rb

@@ -62,6 +62,14 @@ def create_table(*args, &block)
     t.string :name
   end
 
+  create_table :articles, force: true do |t|
+  end
+
+  create_table :articles_magazines, force: true do |t|
+    t.references :article
+    t.references :magazine
+  end
+
   create_table :audit_logs, force: true do |t|
     t.column :message, :string, null: false
     t.column :developer_id, :integer, null: false
@@ -385,6 +393,9 @@ def create_table(*args, &block)
     t.column :custom_lock_version, :integer
   end
 
+  create_table :magazines, force: true do |t|
+  end
+
   create_table :mateys, id: false, force: true do |t|
     t.column :pirate_id, :integer
     t.column :target_id, :integer