Optimize Rails eager loading query for find all

Question

In a Rails 2.3.5 application I've got something like the following models:

class Foo < ActiveRecord::Base
  has_many :bars
end

class Bar < ActiveRecord::Base
  belongs_to :foo
end

And when I'm calling

Foo.all(:include => :bars)

I see the following queries in console:

 SELECT * FROM "foos"
 SELECT "bars".* FROM "bars" WHERE ("bars".foo_id IN (1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21))

with all the foo's ids in the where clause.

I guess this is not an optimal query while the number of ids may be large, and I need to preload all the 'bars'. Also, actually I've got not two models, but a chain of them.

Is there a way to make the eager loading query be like

SELECT "bars".* FROM "bars" 

when I'm using find all?

Answer 1

That's actually an optimization, in fact Rails changes the querying strategy if the number of id's goes high.

You could also use :join instead of using :include