Question

Is there a way to do a second query and have that replicate the functionality of eager loading? Lets say a real basic example. A Person has many Cars and a Car has many Parts..

Typically to eager load you would say:

person.cars.includes(:parts)

I don’t really understand what this does.. does it just load those other objects in memory? Are they associated with each car at this point in a more substantial way then just sharing keys? If its just sharing keys.. could I load these ‘Parts’ in memory and have them accessed without calling the ‘includes’ explicitly? For example.. something like:

cars = person.cars
cars_id_array << insert cars ids here
parts = Parts.where(:user_id => person_id, "car_id IN cars_id_array") (just an example)

In this case (assuming i did the write code which I’m sure I didn’t.. could I go car1.parts, car2.parts and would those parts be already loaded in memory?

If not, what is eager loading doing?

Answer 1

Eager loading is doing something very similar to what you are doing, but in addition to simply querying the database, it’s also establishing relationships. When you retrieve a model from the database that has related models, the related models are generally loaded on demand. The example from the guides is:

clients = Client.limit(10)

clients.each do |client|
  puts client.address.postcode
end

When you do the first query, there’s a call to the database to grab ten clients. As you iterate through them to get the postcode that is associated with a client address (which presumably lives in a different table), you’ll need to do 10 more queries to grab the proper addresses. By using eager loading, you can do the whole thing in just two queries (which are handled under the covers).

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

This grabs the ten clients, then does another query (similar to yours) to grab the addresses that are in the array of client_id’s. These are then resident in memory, with relationships in tact, so you don’t need to make another database call to get them.

If you tried to replicate this by just loading them seperately:

clients = Client.limit(10)
addresses = Address.where("client_id in [my ID array]")
clients.each do |client|
  puts client.address.postcode
end

You’re still going to be hitting the database for each of the addresses, since those objects aren’t associated with each other except through the database id’s. The objects are loaded, but the relationships aren’t created between them.