让我们说我有一个课程，学生可以通过会员注册（课程与学生例如，一个has_and_belongs_to_many relationsip）。有些成员是学生谁是刚刚观察类（不适用于信用卡等），因此：

Let's say I have a Course in which Students can enroll via a Membership (e.g. a has_and_belongs_to_many relationsip of Courses and Students). Some memberships are for students who are just observing the class (not for credit, etc.), so:

class Course < ActiveRecord::Base
  has_many :memberships

  has_many :students,
           :through => :memberships

  has_many :observers,
           :through => :memberships,
           :source => :student,
           :conditions => { :memberships => { :observer => true }}
end

下面就是伟大工程：

observers = Course.find(37).observers

下面就是不起作用：

new_observer = Course.find(37).observers.build(:name => 'Joe Student')

我本来以为，人们可以使用该协会建立新的记录，以及将产生的：

I would have thought that one could build new records using the association and that would have generated:

而是我得到：

ActiveRecord::AssociationTypeMismatch: Membership expected, got Array

我敢说，我完全糊涂了有关如何以及将AP preciate任何见解！我也试着用对会员制模式命名范围做到这一点，但我似乎无法得到的has_many使用范围，该协会。

I'm sure I'm totally confused about how this and would appreciate any insights! I've also tried to do this with named scopes on the Membership model, but I can't seem to get has_many to use a scope in the association.

非常感谢您的帮助可能的！

Thanks so much for any help possible!

推荐答案

我相信你已经遇到了Rails的错误。我想同样的事情在我的盒（2.3.4），它给了我同样的错误，这似乎不正确的。此外，我也试过围绕工作：

I believe that you have encountered a Rails bug. I tried the same thing on my box (2.3.4) and it gives me the same error which doesn't seem right at all. Additionally I also tried the work around of:

course = Course.first
course.observers << Student.create(:name => "Joe Student")
course.save

但是，这将创建一个与观测场集中的成员为错误！

最后一个丑陋的解决办法，我想出了手动创建成员记载：

The final ugly workaround I came up with was creating the Membershiprecord manually:

Membership.create!(:course => Course.first, :student => Student.first, :observer => true)

我有created为此一票，我会在早餐后进一步调查。

I've created a ticket for this and I'll be investigating further after breakfast.

修改：我已经按照承诺，进一步研究发现，如果你改变你的：条件散到数组，如：

EDIT: I have, as promised, investigated further and found if you change your :conditions Hash to an Array such as:

:conditions => ["memberships.observer = ?", true]

它按预期工作。我也有一个 GitHub的资源库，提供例如code和指令重复。

It works as intended. I also have a github repository with example code and instructions to duplicate.