Question

How do I select all child rows corresponding to a certain parent table using ActiveRecord queries.

create table foo (id, ...)
create table bar (id, foo_id, ...)

select * from foo where key = ? 
# The above return a single row or multiple rows.

select * from bar where foo_id in <ids from the above output>

How do I achieve this?

Answer 1

Using only queries with ActiveRecord

foo = Foo.where(:key = ?)
Bar.where(:foo_id => foo)

Using associations and eager loading :

#foo model
class Foo < ActiveRecord::Base
  has_many :bars
end

#bar model
class Bar < ActiveRecord::Base
 belongs_to :foo
end

#query
Foo.includes(:bars).where(:key => ?).collect(&:bars)