Eloquent Computed Attributes

Automatically compute attributes on Laravel Eloquent models when their input changes

Computed attributes provide an alternative to Laravel's built-in accessors. In contrast to accessors, computed attributes are not computed when their value is requested, but rather when their dependencies change (computed attributes are persisted in the DB). It should be preferred in cases of resource intensive computations.

It works by listening for a models' saving event and checking whether the dependencies of a computed attribute are dirty. If so, the attribute is recomputed and its new value is updated in the database.

Dependencies of a computed attribute are declared through the signature of the "compute" method. Argument names in that method correspond to attributes on the model. See below for an example on how this works.

Example Use Cases:

• Render markdown fields as html
• Save excerpts of longer texts
• Geocoding of addresses on update

Comparison with other options:

Method	Use Case	Computation time
Laravel Accessors	Simple transformations (affected by n+1 issue)	On read
laravel-computed-properties	Querieable attributes that should be computed on read (avoids n+1 issue).	On read
This package	Time consuming operations or operations involving external API calls	On write

Getting Started

1. Installation

composer require sehrgut/eloquent-computed-attributes

2. Define a computed attribute

Let's say we have a Post model with a text column and a text_excerpt column. Every time the value of text changes, we want to recompute the text_excerpt column. This is what our Post class could look like:

<?php

namespace App\Models;

use Illuminate\Database\Eloquent\Model;
use SehrGut\EloquentComputedAttributes\HasComputedAttributes;

class Post extends Model
{
    use HasComputedAttributes;

    /** @inheritDoc */
    protected $fillable = ['title', 'text'];

    /**
     * Recompute the `text_excerpt` attribute when `text` changes.
     *
     * @param  string $text
     * @return string
     */
    public function computeTextExcerptAttribute(string $text): string
    {
        return substr($text, 0, 255) . '…';
    }
}

What's happening under the hood:

1. The HasComputedAttributes trait recognises that by defining computeTextExcerptAttribute, we want to compute a text_excerpt attribute (following the naming convention of Accessors & Mutators)
2. The trait derives the computed attributes' dependencies from the method signature: If $this->text changes ($text being part of the method signature), text_excerpt needs to be recomputed
3. On each saving event of the model, if any of the computed attributes' dependencies (arguments of the method) have changed, the method will be called with the updated values as arguments, and its return value will be assigned to $this->text_excerpt before the models' save() method is called.