Eager Loading Associations in rails

Eager loading is the mechanism for loading the associated records of the objects returned by Model.find using as few queries as possible.

N + 1 queries problem

Consider the following code, which finds 10 clients and prints their postcodes:

clients = Client.all(:limit => 10)

clients.each do |client|

  puts client.address.postcode

end

This code looks fine at the first sight. But the problem lies within the total number of queries executed. The above code executes 1 ( to find 10 clients ) + 10 ( one per each client to load the address ) = 11 queries in total.

This is possible by specifying the includes method of the Model.find call

Revisiting the above case, we could rewrite Client.all to use eager load addresses:

clients = Client.includes(:address).limit(10)

clients.each do |client|

  puts client.address.postcode

end

The above code will execute just 2 queries, as opposed to 11 queries in the previous case:

SELECT * FROM clients LIMIT 10

SELECT addresses.* FROM addresses

  WHERE (addresses.client_id IN (1,2,3,4,5,6,7,8,9,10))