Ruby on Rails

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
http://guides.rubyonrails.org/active_record_querying.html , 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?

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