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 can simply say:

Customer.includes(:purchases).all.select{|c| c.purchases.count > 2} 

Effectively though, the line above makes query on the magnitude of Customer.all and Purchase.all, then does the "select" type processing in ruby. In a large database, I would much prefer to avoid doing all this "select" calculation in ruby, and have mysql do the processing and only give me the list of qualified customers. That is both much faster (since mysql is more tuned to do this) and significantly reduces bandwidth from the database.

Unfortunately I am unable to conjure up the code with the building blocks in rails(where, having, group, etc) to make this happen, something on the lines of (psudo-code):

Customer.joins(:purchases).where("count(purchases) > 2").all 

I will settle for straight MySql solution, though I much prefer to figure this out in the elegant framework of rails.

Answer 1

No need to install a gem to get this to work (though metawhere is cool)

Customer.joins(:purchases).group("customers.id").having("count(purchases.id) > ?",0) 

Answer 2

The documentation on this stuff is fairly sparse at this point. I'd look into using Metawhere if you'll be doing any more queries that are similar to this. Using Metawhere, you can do this (or something similar, not sure if the syntax is exactly correct):

Customer.includes(:purchases).where(:purchases => {:count.gte => 2}) 

The beauty of this is that MetaWhere still uses ActiveRecord and arel to perform the query, so it works with the 'new' rails 3 way of doing queries.

Additionally, you probably don't want to call .all on the end as this will cause the query to ping the database. Instead, you want to use lazy loading and not hit the db until you actually require the data (in the view, or some other method that is processing actual data.)