Count has_many records of query

Question

If I have a Model with a has_many relationship, how can I retrieve all of the records that all of the records in my query point to?

Let's just say, buildings have a has_many relationship with rooms. Here's what I want to do:

Building.where(...query...).rooms.count

This is just an example. I might want to count them, or I might want an ActiveRecord of the rooms that belong to the buildings that match the query.

One way is this, but I'm wondering if there's a better way:

building_ids = Building.where(...query...).pluck(:id)
Room.where(building_id: building_ids).count

Answer 1

Using select instead of pluck will result in a single sql statement instead of two separate ones.

building_ids = Building.where(...).ids
Room.where(building_id: building_ids)

you can also use join

Room.joins(:building).where(building: { name: 'somename' })

Answer 2

I'd use the sum of the counter caches.

Add the counter cache colum in a migration:

add_column :buildings, :rooms_count, default: 0

Update the relation in Room:

belongs_to :building, counter_cache: true

Then you could do something like:

Building.sum(:rooms_count)

That'll avoid n+1 queries

More details here https://blog.appsignal.com/2018/06/19/activerecords-counter-cache.html