Arel average of averages

Lets say I have a few of tables

orders = Arel::Table.new :orders
stores = Arel::Table.new :stores
managers = Arel::Table.new :managers

And a manager has many stores and a store has many orders.

One day I want to query for the average total across orders where a
manager works. Oh, I want to group that by the store. So to be clear,
I want to get the average order total for a manager, for each of the
stores they work at.

And let’s assume we’ve looked up our manager:

manager = Manager.find(some_id)

totals =
orders.where(orders[:store_id].in(manager.store_ids)).group(orders.store_id).project(orders[:total].average)

puts totals.to_sql

“SELECT AVG(orders.total) AS avg_id FROM orders WHERE
orders.store_id IN (1, 2, 3) GROUP BY orders.store_id

Yup, that works great. But how can I get a query for the average of
those averages?

What’s the Arel to get this query?

“SELECT AVG(avg_id) FROM (SELECT AVG(orders.total) AS avg_id FROM
orders WHERE orders.store_id IN (1, 2, 3) GROUP BY
orders.store_id) as avg_id_alias;”

Anybody know?

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