use correct DB connection for generated HABTM table

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

@@ -46,7 +46,7 @@ def through_model
 
       join_model = Class.new(ActiveRecord::Base) {
         class << self;
-          attr_accessor :class_resolver
+          attr_accessor :left_model
           attr_accessor :name
           attr_accessor :table_name_resolver
           attr_accessor :left_reflection
@@ -58,7 +58,7 @@ def self.table_name
         end
 
         def self.compute_type(class_name)
-          class_resolver.compute_type class_name
+          left_model.compute_type class_name
         end
 
         def self.add_left_association(name, options)
@@ -72,11 +72,15 @@ def self.add_right_association(name, options)
           self.right_reflection = _reflect_on_association(rhs_name)
         end
 
+        def self.retrieve_connection
+          left_model.retrieve_connection
+        end
+
       }
 
       join_model.name                = "HABTM_#{association_name.to_s.camelize}"
       join_model.table_name_resolver = habtm
-      join_model.class_resolver      = lhs_model
+      join_model.left_model          = lhs_model
 
       join_model.add_left_association :left_side, anonymous_class: lhs_model
       join_model.add_right_association association_name, belongs_to_options(options)

activerecord/test/cases/associations/has_and_belongs_to_many_associations_test.rb

@@ -3,6 +3,7 @@
 require 'models/computer'
 require 'models/project'
 require 'models/company'
+require 'models/course'
 require 'models/customer'
 require 'models/order'
 require 'models/categorization'
@@ -14,6 +15,7 @@
 require 'models/parrot'
 require 'models/person'
 require 'models/pirate'
+require 'models/professor'
 require 'models/treasure'
 require 'models/price_estimate'
 require 'models/club'
@@ -923,4 +925,14 @@ def test_association_force_reload_with_only_true_is_deprecated
 
     assert_deprecated { developer.projects(true) }
   end
+
+  def test_alternate_database
+    professor = Professor.create(name: "Plum")
+    course = Course.create(name: "Forensics")
+    assert_equal 0, professor.courses.count
+    assert_nothing_raised do
+      professor.courses << course
+    end
+    assert_equal 1, professor.courses.count
+  end
 end

activerecord/test/models/professor.rb

@@ -0,0 +1,5 @@
+require_dependency 'models/arunit2_model'
+
+class Professor < ARUnit2Model
+  has_and_belongs_to_many :courses
+end

activerecord/test/schema/schema.rb

@@ -939,3 +939,12 @@ def except(adapter_names_to_exclude)
 College.connection.create_table :colleges, force: true do |t|
   t.column :name, :string, null: false
 end
+
+Professor.connection.create_table :professors, force: true do |t|
+  t.column :name, :string, null: false
+end
+
+Professor.connection.create_table :courses_professors, id: false, force: true do |t|
+  t.references :course
+  t.references :professor
+end

activerecord/test/support/connection.rb

@@ -1,6 +1,7 @@
 require 'active_support/logger'
 require 'models/college'
 require 'models/course'
+require 'models/professor'
 
 module ARTest
   def self.connection_name