Rails has_many :through 和 :conditions 。如何创建关联？

Question

我有以下通过条件与 has_many 关联的模型。

（注意Membership 验证kind 属性的存在）

class User < ActiveRecord::Base
  has_many :memberships
  has_many :founded_groups,
    :through => :memberships,
    :source  => :group,
    :class_name => 'Group'
    :conditions => {'memberships.kind' => 'founder'}
  has_many :joined_groups, ... # same as above, but the kind is 'member'
end

class Group < ActiveRecord::Base
  has_many :memberships
  has_many :founders, ...        # these two mirror the User's
  has_many :regular_members, ... #
end

class Membership < ActiveRecord::Base
  validates_presence_of :user_id
  validates_presence_of :club_id
  validates_presence_of :kind   # <-- attention here!

  belongs_to :user
  belongs_to :group
end

Rails 似乎喜欢上面的代码（至少它不喜欢它）。但后来发生了这种情况：

> user = User.create(...) # valid user
> club = Club.create(...) # valid club
> user.founded_clubs = [club]

ActiveRecord::RecordInvalid: Validation failed: kind can't be blank

> club.founders << user

ActiveRecord::RecordInvalid: Validation failed: kind can't be blank

我假设 rails 会采用我的代码的 {'memberships.kind' => 'founder'} 部分并在创建关联时使用它，但似乎并非如此。所以新成员的kind 是空白的，这会引发错误。

有没有一种简单的方法来创建关联，而不是完全痛苦？

Answer 1

这肯定会起作用：

> user = User.create(...) # valid user
> club = Club.create(...) # valid club
> user.memberships.create(:club_id => club.id, :kind => 'founder')

我不确定，但这可能有效：

> user.memberships.create(:club => club, :kind => 'founder')