gistfile1.rb

require 'active_record'

ActiveRecord::Base.establish_connection(
  adapter: 'sqlite3',
  database: 'test',
)

class Migration < ActiveRecord::Migration
  def up
    drop_table :users rescue nil
    create_table :users do |t|
      t.integer :age
    end

    drop_table :foos rescue nil
    create_table :foos do |t|
      t.integer :user_id
    end
  end
end

Migration.new.up

class User < ActiveRecord::Base
  has_many :foos
end

class Foo < ActiveRecord::Base
  belongs_to :user
end

users = 100.times.map { User.create(age: rand(100)) }
foos = 1000.times.map { Foo.create(user: users.sample) }

ActiveRecord::Base.logger = Logger.new $stdout
p Foo.joins(:user).where(user_id: User.where('users.age % 3 = 0')).group(:user_id).count