Rails activerecord order by field in related table
I have a typical forum style app. There is a Topics
model which has_many
Posts
.
What I want to do using Rails 2.3.x is query the topics table and sort by the most recent post in that topic.
@topics = Topic.paginate :page => params[:page], :per_page => 25,
:include => :posts开发者_开发百科, :order => 'HELP'
I'm sure this is a simple one but no joy with Google. Thanks.
Sorting on a joined column is probably a bad idea and will take an enormous amount of time to run in many situations. What would be better is to twiddle a special date field on the Topic model when a new Post is created:
class Post < ActiveRecord::Base
after_create :update_topic_activity_at
protected
def update_topic_activity_at
Topic.update_all({ :activity_at => Time.now }, { :id => self.topic_id})
end
end
Then you can easily sort on the activity_at
column as required.
When adding this column you can always populate the initial activity_at
with the highest posting time if you have existing data to migrate.
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