Eager loading associations with ordering

This was a sneaky bug that I managed to track down in my code… I hope
it helps someone. I first thought this might be a shortcoming or a bug
in Rails but I’m not really sure where the fault lies: I guess it’s
just an unexpected or suprising behaviour, or else my misuse of
ActiveRecord’s magic.

Problem simplified down to the ubiquitous Project/Task paradigm.

Let’s use acts_as_list for convenience.

class Project < ActiveRecord::Base
has_many :tasks, :order => :position

def next_action
tasks.first
end
end

class Task < ActiveRecord::Base
belongs_to :project
acts_as_list :scope => :project_id
end

Let’s create a project with some tasks and move

the last one to the top, as might happen in normal use:

p = Project.create :name => “Halloween costume”
task_names = [“paint wings”, “recharge lightsaber”, “get bear slippers”]
task_names.each do |t|
p.tasks << Task.new :name => t
end
p.tasks.last.move_to_top

Now in our controller, we want to select all projects ordered by name.

We’ll also want to display the next action… so let’s

eager load the tasks association:

@projects = Project.find :all, :order => :name, :include => :tasks

in the view:

<% @projects.each do |project| %>
<%= project.next_action %>
<% end %>

Surprise! the next action is not the one we moved to the top! What I’m
understanding is that the find action uses only :order => :name, and
replaces the position ordering which is specified in the model.

Thoughts:

Could Rails handle this more intelligently for us? I had expected that
it would pile on the :order statements like “ORDER BY project.name,
task.position” but it doesn’t seem to do that at all.

Or else, should rails put up a warning when something like this occurs
(when an existing ordering is overwritten)?

To work around this, Instead of the method above, I added a proxy on
the Project, replacing the next_action method above:

has_one :next_action, :class_name => ‘Task’, :conditions => {
:position => 1 }

This seems to work, and I can now include just the next_action
association where needed. Problem solved. But, this works because we’re
no longer using a has_many association at all. I’m still wondering if
it’s possible to maintain multiple ordering with has_many… Any
thoughts on this?

–Andrew V.

Could Rails handle this more intelligently for us? I had expected that
it would pile on the :order statements like “ORDER BY project.name,
task.position” but it doesn’t seem to do that at all.

You’re correct, it’s ignored. The eager loading code is basically
hellish enough as it is. Assuming it did work, you might also have
problems if for whatever reason the associations you were eager
loading required conflicting orderings (and it might not be
completely obvious to work out this was the case). I’ve actually got
a patch (#9640) in trac currently that reimplements :include in such
a way that a lot of things (including order) work a lot nicer, but
it’s still a work in progress

Or else, should rails put up a warning when something like this occurs
(when an existing ordering is overwritten)?
Quite possibly.

Fred

This forum is not affiliated to the Ruby language, Ruby on Rails framework, nor any Ruby applications discussed here.

| Privacy Policy | Terms of Service | Remote Ruby Jobs