我如何使用ActiveRecord通过中介模型对象有条件地获取一些模型的对象及其关联的模型的对象

Question

How can I get some model's objects and its associated model's objects conditionally through an intermediary model object? This will then be used to generate JSON (via to_json ).

Here is the setup made more generic:

class JobSet < ActiveRecord::Base
  belongs_to :job_system
  has_many :resources, through => :job_system
  has_many :jobs
  ...
end

class JobSystem < ActiveRecord::Base
  has_many :job_sets
  has_many :resources
  ...
end

class Resource < ActiveRecord::Base
  belongs_to :job_system
  has_many_and_belongs_to_many :jobs
  ...
end

class Job < ActiveRecord::Base
  belongs_to :job_system
  belongs_to :job_set
  has_many_and_belongs_to_many :resources
  ...
end

What I want to do is get all of the resources with their corresponding jobs, but only the jobs that are part of a particular job set.

UPDATE : even if a resource doesn't have any jobs associated with it I still want to get it with the rest of the resources. The purpose is to display all resources with any jobs assigned to them.

It looks like I can set conditions when using through , but I can't seem to figure out how to do that when it is the next objects associations I am after...

Is there a good name for this type of problem? Is there any easy way to do this?

UPDATE 2 : There seem to be solutions that work well within the controller, but I am using to_json in the view to preload for a canvas element. It seems to automatically include all jobs when I do this:

var resources = <%= @resources.to_json(:include => {:jobs => {:only => :id}}) %>; 

Would a manual SQL JOIN even help with this?

Answer 1

I think I know what you are after. The only thing I would ask you for is, if a resource does not have any jobs in this particular job set, should it still be included?

Anyway, here is one way to do it:

@resources = Resource.all(:include => :jobs,
                          :conditions => ["jobs.job_set_id = ?", @job_set.id])

This will NOT include the resources that has no jobs associated with @job_set

So, am I totally wrong in my assumptions, or was this what you where after?

Edit

Okay, this was more difficult then I thought. There are several solutions but none I really like. You could do like this:

@resources = Resource.all

@resources.each do |resource|
  resource.jobs.all(:conditions => ["jobs.job_set_id = ?", @job_set_id])
end

But that will result in the 1+n number-of-queries-problem putting pressure on your db.

You could also do like this:

@resources = Resource.all(:include => :jobs)

@resources.each do |resource|
  resource.jobs.select{ |job| job.job_set_id == @job_set_id }
end

But that will result in more memory consumption on the server side since all jobs are loaded eagerly and then looped through to see which matches the job_set_id.

The problem with finding a better solution in my opinion is that this is something that should be specified in the SQL JOIN condition but I don't think that ActiveRecord lets us modify that in combination with eager loading.

Answer 2

You should be able to do this in a pretty straight-forward way, but you may need the set in a particular format or structure other than what's given here:

@resources = @job_set.resources.include(:jobs)

As a note, I'd advise against using the Rails 1 style has_many_and_belongs_to_many declarations. The join tables they use are not model based and are very hard to manipulate, especially if they have meta-data associated with them, compared to a more modern join model approach. You simply make a model called JobResource or ResourceJob depending on how you want to prioritize things, and build in two belongs_to relationships to join them together.