如何HAS_ONE：通过工作？ [英] How does has_one :through work?

问题描述

我有三个型号：

class ReleaseItem < ActiveRecord::Base
  has_many :pack_release_items
  has_one :pack, :through => :pack_release_items
end

class Pack < ActiveRecord::Base
  has_many :pack_release_items
  has_many :release_items, :through=>:pack_release_items
end

class PackReleaseItem < ActiveRecord::Base
  belongs_to :pack
  belongs_to :release_item
end

的问题是，在执行过程中，如果我添加一个包到一个release_item它不知道该包是一个包。例如：

The problem is that, during execution, if I add a pack to a release_item it is not aware that the pack is a pack. For instance:

Loading development environment (Rails 2.1.0)
>> item = ReleaseItem.new(:filename=>'MAESTRO.TXT')
=> #<ReleaseItem id: nil, filename: "MAESTRO.TXT", created_by: nil, title: nil, sauce_author: nil, sauce_group: nil, sauce_comment: nil, filedate: nil, filesize: nil, created_at: nil, updated_at: nil, content: nil>
>> pack = Pack.new(:filename=>'legion01.zip', :year=>1998)
=> #<Pack id: nil, filename: "legion01.zip", created_by: nil, filesize: nil, items: nil, year: 1998, month: nil, filedate: nil, created_at: nil, updated_at: nil>
>> item.pack = pack
=> #<Pack id: nil, filename: "legion01.zip", created_by: nil, filesize: nil, items: nil, year: 1998, month: nil, filedate: nil, created_at: nil, updated_at: nil>
>> item.pack.filename
NoMethodError: undefined method `filename' for #<Class:0x2196318>
    from /usr/local/lib/ruby/gems/1.8/gems/activerecord-2.1.0/lib/active_record/base.rb:1667:in `method_missing_without_paginate'
    from /usr/local/lib/ruby/gems/1.8/gems/mislav-will_paginate-2.3.3/lib/will_paginate/finder.rb:164:in `method_missing'
    from /usr/local/lib/ruby/gems/1.8/gems/activerecord-2.1.0/lib/active_record/associations/association_collection.rb:285:in `send'
    from /usr/local/lib/ruby/gems/1.8/gems/activerecord-2.1.0/lib/active_record/associations/association_collection.rb:285:in `method_missing_without_paginate'
    from /usr/local/lib/ruby/gems/1.8/gems/activerecord-2.1.0/lib/active_record/base.rb:1852:in `with_scope'
    from /usr/local/lib/ruby/gems/1.8/gems/activerecord-2.1.0/lib/active_record/associations/association_proxy.rb:168:in `send'
    from /usr/local/lib/ruby/gems/1.8/gems/activerecord-2.1.0/lib/active_record/associations/association_proxy.rb:168:in `with_scope'
    from /usr/local/lib/ruby/gems/1.8/gems/activerecord-2.1.0/lib/active_record/associations/association_collection.rb:281:in `method_missing_without_paginate'
    from /usr/local/lib/ruby/gems/1.8/gems/mislav-will_paginate-2.3.3/lib/will_paginate/finder.rb:164:in `method_missing'
    from (irb):5
>>

看来我应该有机会获得item.pack，但不知道包是包项目。

It seems that I should have access to item.pack, but it is unaware that the pack is a Pack item.

推荐答案

看来，您的使用情况HAS_ONE的：通过是正确的。你所看到的问题，是因为有节省的对象。对于协会的工作，也就是被引用的对象需要有一个id来填充 model_id 字段的对象。在这种情况下， PackReleaseItems 有一个 pack_id 和 release_item_id 现场需要填写为协会正常工作。尝试通过节能协会访问对象之前。

It appears that your usage of has_one :through is correct. The problem you're seeing has to do with saving objects. For an association to work, the object that is being referenced needs to have an id to populate the model_id field for the object. In this case, PackReleaseItems have a pack_id and a release_item_id field that need to be filled for the association to work correctly. Try saving before accessing objects through an association.

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