Laravel sync relationship method — a word of caution

First of all Laravel is a great framework. I really enjoy to work with it and it’s my number one tool for developing web applications. However as great as it is, there are some gotchas here and there that can bite you in unexpected way and in the worst time as well.

So, my personal advice to you — learn more about tools you are using. Code review, articles, videos, anything that help you understand what’s going on under the hood at least on a basic level. In more specific cases there is no other way than going through the code itself.

As a bonus you may learn a thing or two from it ;)

During the development with Laravel I’ve been multiple times in such kind of a situations, but here I want to speak about one particular case.

As you probably know, Eloquent has many helper methods to work with relationships. And they do make your life easier… until they don’t.

One of such helpers is a sync() method, or in our case it was syncWithoutDetaching() method. In short it helps you to synchronize a many-to-many relationship between specific model A and group of models B in a way that:

  • the only models B that will be assigned to the model A are the ones in a received group —sync()
  • only the missing models B from the received group will be assigned to model A — syncWithoutDetaching()

Sounds good, right?

We used the syncWithoutDetaching() method for any case we wanted to add a new models to the relationship but only the missing ones. The perfect case for this helper method.

Everything worked just great.

Fast forward a couple of years and suddenly the simple request of attaching a single model to another model fails with an “Out of memory” exception. In a production.

Say “Hello” to the scalability issues!

The code was pretty simple:

That single line consumed an enormous amount of memory!

After the some research I figured out that the reason for that is in a way the sync() method works. I know I was talking about syncWithoutDetaching(), but it’s just an alias for specific use case of sync() method.

What sync() method actually does - it loads all existing pivot identifiers to an array and then starts to make some manipulations on that array and array of received earlier identifiers to attach. With some additional arrays duplication on the way.

Which is fine when you work with a couple of hundreds rows, but not that great when you have to load a couple of hundreds of thousands or even more rows.

So, be careful when you using this helper in your applications, especially when you expect it to scale up drastically.

Several ways to achieve the desired functionality without hitting the memory limit:

  • check the existence of just the models you want to attach and then use the attach() method on the missing ones
  • for a large amounts of models to attach it’s probably better to use jobs and chunk the models
  • creating a primary key on two foreign keys and catching the exceptions from DB (I’ve seen such suggestions, although in my opinion it’s not that convenient when you are trying to attach a large number of models)
  • write some custom sync() method with your own logic
  • work with directly with a DB instead of using eloquent helpers

I’m sure there are dozens of other ways to optimize that. Let me know in the comments what was your use case and how you solved it.

That’s it for today. Thank you for your attention and your time!

Entrepreneur and full-stack web developer