rails - using：select（distinct）with：has_many：通过关联产生无效的SQL

Question

User
has_many :posts
has_many :post_tags, :through => :posts

PostTag
belong_to :post
belongs_to :tag
scope :distincttag, :select => ('distinct post_tags.tag_id')

使用Rails 3.0.4，我得到无效的SQL： SELECT post_tags。*，distinct tag_id ...

至少有一个人遇到了同样的问题：http://www.ruby-forum.com/topic/484938

功能还是错误？

感谢

Answer 1

放置范围看起来不合适。

也许你正在努力实现这个目标：

class PostTag < ...
  belong_to :post
  belongs_to :tag
  def distincttag
    find(:all, :select => 'distinct tag_id')
  end
end

编辑：现在我知道你需要什么了：

User
has_many :posts
has_many :post_tags, :through => :posts, :select => 'distinct tags.*'
# or, if you are not worried about database overhead:
has_many :post_tags, :through => :posts, :uniq => true

参考：http://blog.hasmanythrough.com/2006/5/6/through-gets-uniq