Rails 3，具有 lambda 条件的 has_one/has_many

问题描述

这是我的模型:

class User < ActiveRecord::Base
  has_many :bookmarks
end

class Topic < ActiveRecord::Base
  has_many :bookmarks
end

class Bookmark < ActiveRecord::Base
  belongs_to :user
  belongs_to :topic
  attr_accessible :position
  validates_uniqueness_of :user_id, :scope => :topic_id
end

我想获取所有topics，对于current_user，关联的bookmark.ATM，我愿意:

I want to fetch all topics with, for current_user, the associated bookmark. ATM, I do:

Topic.all.each do |t|
    bookmark = t.bookmarks.where(user_id: current_user.id).last
    puts bookmark.position if bookmark
    puts t.name
end

这很丑陋，而且做了太多的数据库查询.我想要这样的东西:

This is ugly and do too much DB queries. I would like something like this:

class Topic < ActiveRecord::Base
  has_one :bookmark, :conditions => lambda {|u| "bookmarks.user_id = #{u.id}"}
end

Topic.includes(:bookmark, current_user).all.each do |t| # this must also includes topics without bookmark
    puts t.bookmark.position if t.bookmark
    puts t.name
end

这可能吗?我有其他选择吗?

Is this possible? Have I any alternative?

谢谢！

推荐答案

*嗯，我不确定我是否理解您的问题，但这可能对您有所帮助:

*Hmm I'm not sure I understood your question but this may help you:

# code
class Topic < ActiveRecord::Base
  scope :for_user, lambda { |user| includes(:bookmarks).where(bookmarks: { user_id: user.try(:id) || user} ) }

# call
Topic.for_user(current_user) # => Array of Topics

如您所见，for_user 范围的参数 可以是用户对象或用户 ID.

As you can see the parameter of the scope for_user can be a User object OR a user id.

希望这有帮助！

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