Ruby on Rails按子属性命令父模型

Question

How would I go about doing this? I would like to sort a selection parent model by an attribute on it's children. The parent model has a has many relationship with it's child. I'm not sure how to form the ActiveRecord query to get this.

Example: A thread has many posts. I want to grab a collection of threads ordered by the most recent post associated with it.

I've found a few solutions that convert the selection to an array and do a sort on that array, but I need the selection to stay as an ActiveRecord selection, so I can continue to chain queries onto it.

Answer 1

我需要有关数据库布局的更多信息以提供更好的示例，但您可以加入另一个表，然后按该表排序，如下所示：

Thread.joins(:posts).order("posts.created_at").group("threads.id")

Answer 2

For your example, it would be far more efficient to simply have the children touch the parent:

class Thread < ActiveRecord::Base
  has_many :posts
end

class Post < ActiveRecord::Base
  belongs_to :thread,
        touch: true
end

This will change Thread's updated_at column whenever a post is created or changed, so that thread is considered updated at that time. You may need to actually access the child records in a more complicated example, but for ordering by the date of the child posts this saves you having to get the extra records out of the database when you don't actually need information from that table.

Answer 3

By using just plain queries, with no direct iteration over arrays, you can do something like:

Post.joins(:thread).includes(:thread).order("posts.created_at").group("thread_id").take(5) 
# Maybe you can take only a few samples of your collection to show on Last Posted Topics, 
# then you just replace take(5) with the number of posts you want to take.