Rails 3 query on condition of an association's count

Question

In Rails 3 with mysql, suppose I have two models, Customers and Purchases, obviously purchase belongs_to customer. I want to find all the customers with 2 orders or more. I

Answer 1

This is a bit more verbose, but if you want Customers where count = 0 or a more flexible sql, you would do a LEFT JOIN

Customer.joins('LEFT JOIN purchases on purchases.customer_id = customers.id').group('customers.id').having('count(purchases.id) = 0').length