Ruby on Rails

On Apr 2, 3:14 am, jenna_s <jenna.sim...@gmail.com> wrote:
> Hi Peter,
>
> How did you know that?! Yes, indeed there are 3 SQL queries:
>
> Message Load (0.5ms)  SELECT `messages`.* FROM `messages` WHERE
> (from_user_id = 3 AND to_user_id = '2' OR to_user_id = 3 AND
> from_user_id = '2') ORDER BY created_at DESC LIMIT 1
> User Load (0.4ms)  SELECT `users`.* FROM `users` WHERE (`users`.`id` =
> 2)
> User Load (0.5ms)  SELECT `users`.* FROM `users` WHERE (`users`.`id` =
> 3)
>
> Why is that? I really expected one SQL statement. In fact I need one
> SQL statement since I need to serialize and respond to in JSON. How
> can I fix it?

This is how include works by default. The loaded users are wired up to
the message objects for you. There is one query per association, so
you get the most benefit when there are multiple messages. The fact
that it takes multiple queries (or even whether you use :include at
all) has no effect on what the serialised json will look like.

You can force AR to use a joins based include strategy via eager_load
if you really want it to. (but like i said whether stuff is eagerly
loaded or not doesn't impact the generated json)

Fred

>
> Thanks so much,
> Jenna
>
> On Mar 31, 1:56 pm, Peter Vandenabeele <pe...@vandenabeele.com> wrote:
>
>
>
>
>
>
>
> > On Sat, Mar 31, 2012 at 10:28 PM, jenna_s <jenna.sim...@gmail.com> wrote:
> > > Hi,
>
> > > I'm having trouble with a simple users-messages setup, where users can
> > > send messages to other users. Here's my class definition:
>
> > > class Message < ActiveRecord::Base
> > >  belongs_to :from_user, :class_name => 'User'
> > >  belongs_to :to_user, :class_name => 'User'
> > > end
>
> > > class User < ActiveRecord::Base
> > >  def messages(with_user_id)
> > >    Message.where("from_user_id = :id AND to_user_id = :with_user_id
> > > OR to_user_id = :id AND from_user_id = :with_user_id",
> > >      { :id => id, :with_user_id => with_user_id })
> > >      .includes(:from_user, :to_user)
> > >      .order("created_at DESC")
> > >  end
> > > end
>
> > > When I do some_user.messages(another_user.id), I want to retrieve the
> > > conversation that some_user has with another_user. I do get back an
> > > array of Messages, but it doesn't include the eager loading of
> > > from_user and to_user.
>
> > How many SQL queries do you see in the log for
>
> > messages = some_user.messages(another_user.id)
>
> > (I believe you should see 3 queries if from_user and to_user data
> > is present).
>
> > When asking later on:
>
> > messages.first.from_user
>
> > is it then launching an SQL for the individual from_user
> > at that time?
>
> > > What am I doing wrong? Am I forced to use
> > > joins?
>
> > It must be possible to get this working with .includes
> > as you tried.
>
> > HTH,
>
> > Peter

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