Required files
active_record/associations/association_proxy
active_record/associations/association_collection
active_record/associations/belongs_to_association
active_record/associations/belongs_to_polymorphic_association
active_record/associations/has_one_association
active_record/associations/has_many_association
active_record/associations/has_many_through_association
active_record/associations/has_and_belongs_to_many_association
active_record/associations/has_one_through_association