PostgreSQL, Rails and :order => problem

问题

I have the following line in my ActiveRecord model:

class Record < ActiveRecord::Base
    has_many :users, :through => :record_users, :uniq => true, :order => "record_users.index ASC"

This is intended to enable me to read out record.users in a way that I order using an index field in the record_users model.

The problem is that this fails on PostgreSQL with the following error:

ActionView::TemplateError (PGError: ERROR:  for SELECT DISTINCT, ORDER BY expressions must appear in select list

Is there a way to fix the statement to make it work?

回答1:

I suppose you could call it a bug in ActiveRecord. PosgreSQL is a bit more restrictive than MySQL. You can help out ActiveRecord by setting up the association like this instead:

class Record < ActiveRecord::Base
  has_many :users,
   :through => :record_users,
   :select => 'DISTINCT users.*, record_users.index',
   :order => "record_users.index ASC"

回答2:

Just posted this issue on rails's issue tracker on github (copied from lighthouse ticket so we could bring it back.. it was marked invalid):

https://github.com/rails/rails/issues/520

Promote it if you want this fixed nicely! :)

回答3:

I ran into something similar before and I believe it's an AR bug related to PGSQL (https://rails.lighthouseapp.com/projects/8994/tickets/1711-has-many-through-association-with-order-causes-a-sql-error-with-postgresql).

I got around it by dropping the DISTINCT (:uniq) directive and resolved the uniq records another way. Kind of a bummer though.

来源：https://stackoverflow.com/questions/1714736/postgresql-rails-and-order-problem