set a constant for the "anonymous" habtm model fixes #15022

I wanted the middle model to be anonymous, but we have to give it a name
for marshal support. :'(

activerecord/lib/active_record/associations.rb

@@ -1576,6 +1576,11 @@ def has_and_belongs_to_many(name, scope = nil, options = {}, &extension)
 
         join_model = builder.through_model
 
+        # FIXME: we should move this to the internal constants. Also people
+        # should never directly access this constant so I'm not happy about
+        # setting it.
+        const_set join_model.name, join_model
+
         middle_reflection = builder.middle_reflection join_model
 
         Builder::HasMany.define_callbacks self, middle_reflection

activerecord/test/cases/associations/has_and_belongs_to_many_associations_test.rb

@@ -83,6 +83,12 @@ def setup_data_for_habtm_case
     country.treaties << treaty
   end
 
+  def test_marshal_dump
+    post = posts :welcome
+    preloaded = Post.includes(:categories).find post.id
+    assert_equal preloaded, Marshal.load(Marshal.dump(preloaded))
+  end
+
   def test_should_property_quote_string_primary_keys
     setup_data_for_habtm_case