Should I drop a polymorphic association?

1
votes

My code is still just in development, not production, and I'm hitting a wall with generating data that I want for some views.

Without burying you guys in details, I basically want to navigate through multiple model associations to get some information at each level. The one association giving me problems is a polymorphic belongs_to. Here are the most relevant associations

Model Post
  belongs_to :subtopic
  has_many :flags, :as => :flaggable

Model Subtopic
  has_many :flags, :as => :flaggable

Model Flag
  belongs_to :flaggable, :polymorphic => true

I'd like to display multiple flags in a Flags#index view. There's information from other models that I want to display, as well, but I'm leaving out the specifics here to keep this simpler.

In my Flags_controller#index action, I'm currently using @flags = paginate_by_sql to pull everything I want from the database. I can successfully get the data, but I can't get the associated model objects eager-loaded (though the data I want is all in memory). I'm looking at a few options now:

  • rewrite my views to work on the SQL data in the @flags object. This should work and will prevent the 5-6 association-model-SQL queries per row on the index page, but will look very hackish. I'd like to avoid this if possible

  • simplify my views and create additional pages for the more detailed information, to be loaded only when viewing one individual flag

  • change the model hierarchy/definitions away from polymorphic associations to inheritance. Effectively make a module or class FlaggableObject that would be the parent of both Subtopic and Post.

I'm leaning towards the third option, but I'm not certain that I'll be able to cleanly pull all the information I want using Rails' ActiveRecord helpers only.

I would like insight on whether this would work and, more importantly, if you you have a better solution

EDIT: Some nit-picky include behavior I've encountered

@flags = Flag.find(:all,:conditions=> "flaggable_type = 'Post'", :include => [{:flaggable=>[:user,{:subtopic=>:category}]},:user]).paginate(:page => 1)

=> (valid response)


@flags = Flag.find(:all,:conditions=> ["flaggable_type = 'Post' AND 
  post.subtopic.category_id IN ?", [2,3,4,5]], :include => [{:flaggable=>
  [:user, {:subtopic=>:category}]},:user]).paginate(:page => 1)

=> ActiveRecord::EagerLoadPolymorphicError: Can not eagerly load the polymorphic association :flaggable
2
ruby-on-rails-3inheritancepolymorphic-associationseager-loadingwill-paginate

2 Answers

1
votes

Don't drop the polymorphic association. Use includes(:association_name) to eager-load the associated objects. paginate_by_sql won't work, but paginate will.

@flags = Flag.includes(:flaggable).paginate(:page => 1)

It will do exactly what you want, using one query from each table.

See A Guide to Active Record Associations. You may see older examples using the :include option, but the includes method is the new interface in Rails 3.0 and 3.1.

Update from original poster:

If you're getting this error: Can not eagerly load the polymorphic association :flaggable, try something like the following:

Flag.where("flaggable_type = 'Post'").includes([{:flaggable=>[:user, {:subtopic=>:category}]}, :user]).paginate(:page => 1)

See comments for more details.

1
votes

Issues: Count over a polymorphic association.

@flags = Flag.find(:all,:conditions => ["flaggable_type = 'Post' AND post.subtopic.category_id IN ?",
[2,3,4,5]], :include => [{:flaggable => [:user, {:subtopic=>:category}]},:user])
.paginate(:page => 1)

Try like the following:

@flags = Flag.find(:all,:conditions => ["flaggable_type = 'Post' AND post.subtopic.category_id IN ?",
[2,3,4,5]], :include => [{:flaggable => [:user, {:subtopic=>:category}]},:user])
.paginate(:page => 1, :total_entries => Flag.count(:conditions => 
["flaggable_type = 'Post' AND post.subtopic.category_id IN ?", [2,3,4,5]]))