Active Record: Eager Loading (syntax question)

rubyonrails-talk
1

Let's have model :parents and :children, where one parent can have many children, and that I want to do something like this:

plist=Parent.joins(:children).select('*').where("parents.id=children.parent_id and children.cfield=#{...}")

plist.each { |p| do_something_with(p,p.children) }

Now I learned from Active Record Query Interface — Ruby on Rails Guides , that this is inefficient, due to the SELECT statements generated inside the block, and that I should do "eager loading" instead. From my understanding of the tutorial, I should replace 'joins' by 'includes':

plist=Parent.includes(:children).select('*').where("parents.id=children.parent_id and children.cfield=#{...}")

However, this raises the exception that there would be no column "children.cfield".

It seems that with 'includes', we can only query based on values of the Parent table.

Is this correct?

2

In rails 4.1 (or possibly 4.0) and later you must use references if you are adding conditions on one of the joined columns, ie

Parent.includes(:children).references(:children)

So that rails knows it must use the joins based eager loading strategy (rails used to try and guess this for you but this was rather error-prone). You also don’t need the select(‘*’)

Fred