How to join on subqueries using ARel?

Question

I have a few massive SQL request involving join across various models in my rails application. A single request can involve 6 to 10 tables.

To run the request faster I w

Answer 1

Was looking for this, and was helped by the other answers, but there are some error in both, e.g. A.join(... should be a.join(....
And I also missed how to build an ActiveRecord::Relation.

Here is how to build an ActiveRecord::Relation, in Rails 4

a = A.arel_table
b = B.arel_table

subsel = B.select(b[:a_id]).where(b[:c].gt('4')).as('sub_select')
joins  = a.join(subsel).on(subsel[:a_id].eq(a[:id])).join_sources
rel    = A.joins(joins)
rel.to_sql
#=> "SELECT `a`.* FROM `a` INNER JOIN (SELECT `b`.`a_id` FROM `b` WHERE (`b`.`c` > 4)) sub_select ON sub_select.`a_id` = `a`.`id`"