Understanding Laravel's SerializesModels trait
Tags: laravel, php
Published: 2021-05-01 17:51:26

_🔗 This article was originally posted, with additional formatting, on my personal blog at https://ryanc.co/blog/understanding-laravels-serializesmodels_

## Background

When dispatching an object onto the queue, behind the scenes Laravel is recursively serializing the object and all of its properties into a string representation that is then written to the queue. There it awaits a queue worker to retrieve it from the queue and unserialize it back into a PHP object (*Phew*!). 

## Problem

When complicated objects are serialized, their string representations can be atrociously long, taking up unnecessary resources both on the queue and application servers. 

## Solution

Because of this, Laravel offers a trait called `SerializesModels` which, when added to an object, finds any properties of type `Model` or `Eloquent\Collection` during serialization and replaces them with a plain-old-PHP-object ([POPO](https://stackoverflow.com/a/41188144)) known as a `ModelIdentifier`. These identifier objects represent the original properties `Model` type and ID, or IDs in the case of an `Eloquent\Collection`, with a much smaller string representation when serialized. When these objects are unserialized, the `ModelIdentifier`s are then replaced with the `Model` or `Eloquent\Collection` of `Model`s that they temporarily represented.

> ℹ️ Curious to know how the `SerializesModels` trait is "replacing" these properties at runtime? Before jumping into the [source code](https://github.com/laravel/framework/blob/8.x/src/Illuminate/Queue/SerializesModels.php), you may want to read the [PHP docs](https://www.php.net/manual/en/intro.reflection.php) for a quick primer on what the reflection API offers. For a more detailed explanation including examples of how Laravel uses reflection check out [this article](https://culttt.com/2014/07/02/reflection-php/).

## Gotcha!

> 🗣 Because of the SerializesModels trait that the job is using, Eloquent models and their loaded relationships will be gracefully serialized and unserialized when the job is processing.

While this quote from the docs sounds promising, it can be misleading. Here is an example of how a `Model` would be represented when serialized.

![https://i.imgur.com/KwnEZsN.png](https://i.imgur.com/KwnEZsN.png)

As you can see, the relation is serialized as `records`, which is the name of a one-to-many relation between `User` and `Record` as it exists on the `User` model. However, even though we're only selecting the `id` and `user_id` columns and have a limit of 3 records to be returned with the `User` model, there's no mention of which records, which properties, or how many in the serialized representation.

Let's see what the query log looks like when we unserialize this object.

![https://i.imgur.com/1oMrVXI.png](https://i.imgur.com/1oMrVXI.png)

*Wow!* We're selecting *all* `Record` models, in their *entirety*, associated to the `User`! As you can probably imagine, this can cause unforeseen issues for the unexpecting artisan.

## Workarounds

Hope is not lost! There are workarounds without having to sacrifice resources on the queue, application, or database servers.

### Unload unnecessary relations

If you don't need the loaded relations to be re-loaded, you can simply call `withoutRelations()` on your `Model` before it's serialized.

![https://i.imgur.com/UZ4j14E.png](https://i.imgur.com/UZ4j14E.png)

As you can see, there are no longer any relations that will be loaded when the `User` model is unserialized.

### Make necessary relations their own property

If the loaded relation is going to be required after the `Model` is unserialized you can store the relation (which is an `Eloquent\Collection`) as its own property on the object being serialized.

![https://i.imgur.com/VnJ1lKY.png](https://i.imgur.com/VnJ1lKY.png)

As you can see, the serialized representation of the `Eloquent\Collection` specifies what the `Model` type is and which IDs need to be retrieved from the database. 

![https://i.imgur.com/WdPdrMl.png](https://i.imgur.com/WdPdrMl.png)

*Much* better! Now that our `Model` and `Eloquent\Collection` of `Record`s have been hydrated, we can even set the relation back on the `Model` by using the `setRelation` method.

## Recap

While it's recommended to use the `SerializesModels` trait on any and all objects that will be queued (or otherwise serialized) it is crucial to be at least aware of its potential pitfalls and shortcomings as well as how to avoid running into them, if not understand how it all works under the hood.

> 🎉 Like the content? Feel free to let me know on [Twitter](https://twitter.com/colson_ryan) where I share all of my articles, packages, and occasional opinions (*mostly* technical).