初学 ruby

------------------------------ model 部分 ------------------------------
class User < ActiveRecord::Base
end

class Member < ActiveRecord::Base
    belongs_to :user
end

class ProjectMember < Member
end

------------------------------ spec 部分 ------------------------------
    factory :project_member do
      association :user, :factory => :user
      project
      access_level { ProjectMember::MASTER }
    end 

此时 p ProjectMember.instance_methods 中明明有 user= 这个函数，但是执行 rspec 却出错

提示 factory_girl 生成假数据时出错 2) project_member factory should be valid
Failure/Error: expect(build(factory_name)).to be_valid

ActiveRecord::AssociationNotFoundError: Association named 'user' was not found on ProjectMember; perhaps you misspelled it?

如果将 ProjectMember 改为

class ProjectMember < Member
  belongs_to :user
end

则一切 OK 我哪里出错了么？