Eager loading of polymorphic associations in ActiveRecord

Question

This is my first time using Rails and I was wondering if it\'s possible to load a has one polymorphic association in one SQL query? The models and associations between them are

Answer 1

(This is for Rails 3 syntax.)

MetaWhere is an awesome gem for making complex queries that are outside of ActiveRecord's usual domain easy and ruby-like. (@wayne: It supports outer joins as well.)

https://github.com/ernie/meta_where

Polymorphic joins are a little trickier. Here's how I did mine with MetaWhere:

Image.joins(:asset.type(AssetModel)).includes(:asset)

In fact, I made a convenience method in my polymorphic-association-having class:

  def self.join_to(type)
    joins(:asset.type(type)).includes(:asset)
  end

So it will always query & eager load a particular type of asset. I haven't tried mixing it with multiple types of join in one query. Because polymorphism works using the same foreign key to reference different tables, on the SQL level you have to specify it as separate joins because one join only joins to one table.

This only works with ActiveRecord 3 because you have access to the amazing power of AREL.