如何检查多态rails has_many集合是否包含x元素

Question

我有四个模型：

User (users)
Caretaker (a completely different type of user)
MessageGroup (a group)
MessageGroupMembership (a join table between MessageGroup and (Users && Caretakers))

我正在尝试定义一种方法来检查是否存在具有 x 个成员的组（成员可以是用户或管理员）。

def group_containing(members)
  # return group if a MessageGroup exists whose members have the members
  # passed in and those member only. No more. No less.
end

我的问题是成员是多态的，所以我无法弄清楚如何最好地构建活动记录查询。

我的课程是：

class User < ActiveRecord::Base
  has_many :message_group_memberships
  has_many :message_groups, through: :message_group_memberships
end

class Caretaker < ActiveRecord::Base
  has_many :message_group_memberships
  has_many :message_groups, through: :message_group_memberships
end

class MessageGroupMembership < ActiveRecord::Base
  belongs_to :member, polymorphic: true
  belongs_to :message_group
end

class MessageGroup < ActiveRecord::Base
  has_many :message_group_memberships
  has_many :members, through: :message_group_memberships
  has_many :messages

  def group_containing(members)
    # method goes here.
  end
end

Answer 1

def group_containing(members)
  member_ids = members.map(&:id)
  MeesageGroup.joins(:message_group_memberships).where({:message_group_memberships => member_ids})
end

试试这个