如何省略voter_memberships关联，仍让voters关联正常工作？

报错原因 #

你写的-> { voter }是作用在User模型的查询集合上的，但voter是Membership模型里定义的enum scope方法，User模型根本没有这个方法，所以会报“undefined local variable or method 'voter'”的错误。

正确写法 #

要直接通过memberships关联筛选出对应角色的用户，有两种可靠方式：

方式一：用merge结合Membership的enum scope（推荐）

利用Rails的merge方法，把Membership模型中定义的enum scope合并到用户查询里，这样既符合enum的最佳实践，后续enum值变化时也不用修改这段代码：

class Team
  belongs_to :owner, class_name: "User"
  has_many :memberships
  has_many :members, through: :memberships, source: :user, class_name: "User"
  
  # 直接通过memberships筛选voter角色的用户
  has_many :voters, -> { merge(Membership.voter) }, through: :memberships, source: :user, class_name: "User"
end

方式二：直接指定where条件

如果觉得merge不够直观，也可以直接在lambda里写where条件，明确指定memberships表的role字段值：

class Team
  belongs_to :owner, class_name: "User"
  has_many :memberships
  has_many :members, through: :memberships, source: :user, class_name: "User"
  
  has_many :voters, -> { where(memberships: { role: :voter }) }, through: :memberships, source: :user, class_name: "User"
end

批量处理其他角色 #

对于contributor、editor、manager这些角色，你可以用完全相同的逻辑批量处理，不用再单独定义xxx_memberships关联：

class Team
  belongs_to :owner, class_name: "User"
  has_many :memberships
  has_many :members, through: :memberships, source: :user, class_name: "User"
  
  has_many :voters, -> { merge(Membership.voter) }, through: :memberships, source: :user, class_name: "User"
  has_many :contributors, -> { merge(Membership.contributor) }, through: :memberships, source: :user, class_name: "User"
  has_many :editors, -> { merge(Membership.editor) }, through: :memberships, source: :user, class_name: "User"
  has_many :managers, -> { merge(Membership.manager) }, through: :memberships, source: :user, class_name: "User"
end

内容的提问来源于stack exchange，提问作者anithri