You can also write a DQL query to fetch both tables at the same time: $dql = "SELECT u, c FROM User u JOIN Country c" Of course, this will be always triggered so you will fetch the country related to a user even when you don't need it. You can decide that a relationship should always be eager-loaded: /** Ruby on Rails is very similar to Eloquent: = (:country)Īnother example: Doctrine ORM comes with several strategies for dealing with eager loading. Eager loading in Eloquentįor instance, Eloquent (the Laravel ORM) comes with a with method: $users = User::with('country')->get() This is more efficient in loading data but it will load data irrespective of the data being used or not. With eager loading, the related data is fetched along with the parent object. The ORM can then fetch that data in advance (we call that "eager loading"). The idea is always the same: the developer should tell in advance to the ORM that it will need additional data. Hopefully, ORM developers have known the issue for quite some time and they already have working solutions. Needless to say the performance of raw SQL is way better than the performance of this hypothetical ORM. Of course, if we were to write pure SQL, we could solve this in exactly one query: SELECT * FROM users JOIN countries ON untry_id = countries.id
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This very simple code is actually performing 2000 + 1 queries! (hence the "N+1" problem). So we will call $user->getCountry().īut we don't have the country data loaded in memory! So the ORM will probably want to fetch the data from the country, by running this query: SELECT * FROM countries WHERE country_id = :country_id_for_current_user Īnd it will do this for each user. Something like: SELECT * FROM users įor each user, we want to fetch the country. On the first line, the ORM will probably fetch all the users. This is PHP but this could really be any other programming language. In this example, each user belongs to a country.Īssuming our ORM gives us a "user repository" that enables us to fetch users, our code could look like this: $users = $userRepository->findAll() Įcho $user->getName().' lives in '.$user->getCountry()->getLabel()."\n" Because you use a nice object model instead of writing SQL, it is easy not to realize that a big number of requests can be emitted. ORMs are tools that write SQL requests for you.
SOLVING THE N+1 PROBLEM IN ORMS THE N+1 PROBLEM
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