ActiveRecord:按属性唯一
我正在尝试筛选ActiveRecord\u AssociationRelations,使其按父id唯一 所以,我想要一个这样的列表:ActiveRecord:按属性唯一,activerecord,ruby-on-rails-4,Activerecord,Ruby On Rails 4,我正在尝试筛选ActiveRecord\u AssociationRelations,使其按父id唯一 所以,我想要一个这样的列表: [#<Message id: 25, posted_by_id: 3, posted_at: "2014-10-30 06:02:47", parent_id: 20, content: "This is a comment", created_at: "2014-10-30 06:02:47", updated_at: "2014-10-30 06:02:
[#<Message id: 25, posted_by_id: 3, posted_at: "2014-10-30 06:02:47", parent_id: 20, content: "This is a comment", created_at: "2014-10-30 06:02:47", updated_at: "2014-10-30 06:02:47">,
#<Message id: 23, posted_by_id: 3, posted_at: "2014-10-28 16:11:02", parent_id: 20, content: "This is another comment", created_at: "2014-10-28 16:11:02", updated_at: "2014-10-28 16:11:02">]}
要返回此文件:
[#<Message id: 25, posted_by_id: 3, posted_at: "2014-10-30 06:02:47", parent_id: 20, content: "This is a comment", created_at: "2014-10-30 06:02:47", updated_at: "2014-10-30 06:02:47">]
我尝试过各种技术,包括:
@messages.uniq(&:parent_id) # returns the same list (with duplicate parent_ids)
@messages.select(:parent_id).distinct # returns [#<Message id: nil, parent_id: 20>]
uniq_by已从Rails 4.1中删除。您尝试过吗
group(:parent_id)
我觉得这就是你想要的。这将返回具有给定父项id的第一个条目。如果您想要最后一个条目,则必须在子查询中对结果重新排序,然后使用组。对于我来说,在Rails 3.2和Postgresql中,Foo.group:bar适用于简单查询,但如果其中有where子句,则会给我一个错误
irb> Message.where(receiver_id: 434).group(:sender_id)
=> PG::GroupingError: ERROR: column "messages.id" must appear in the
GROUP BY clause or be used in an aggregate function
我最后指定了一个SQL“DISTINCT ON”子句来选择。在消息类中,我具有以下作用域:
scope :latest_from_each_sender, -> { order("sender_id ASC, created_at DESC").select('DISTINCT ON ("sender_id") *') }
用法:
irb> Message.where(receiver_id: 434).latest_from_each_sender
谢谢,这正是我想要的。