Custom Eloquent Relations in Laravel Explained | Mohamed Said [Skip to content](#main) [ ![](https://cdn.msaied.com/01KT78WE565VEMM3PSNQAAB0MH.png) Mohamed SaidLaravel Backend Engineer ](https://msaied.com/public) - [Home](https://msaied.com/public) - [Projects](https://msaied.com/public/projects) - [Articles](https://msaied.com/public/articles) - [Certificates](https://msaied.com/public/certificates) - [About](https://msaied.com/public#about) [ Contact](https://msaied.com/public#contact) Menu Menu ---- Close - [HomeStart here](https://msaied.com/public) - [ProjectsCase studies](https://msaied.com/public/projects) - [ArticlesEngineering notes](https://msaied.com/public/articles) - [CertificatesCredentials](https://msaied.com/public/certificates) - [AboutHow I work](https://msaied.com/public#about) - [ContactGet in touch](https://msaied.com/public#contact) [Start a conversation](https://msaied.com/public#contact) [WhatsApp](https://wa.me/201094619204) [Email](mailto:hello@msaied.com) 1. [Home](https://msaied.com/public) 2. / 3. [Articles](https://msaied.com/public/articles) 4. / 5. Eloquent Custom Relations: Building a HasManyThrough Alternative for Complex Joins Eloquent Custom Relations: Building a HasManyThrough Alternative for Complex Joins =================================================================================== Laravel's built-in relations cover most schemas, but complex domains sometimes need a custom relation class. Learn to build one with proper eager loading, match callbacks, and whereHas support from scratch. ![](https://cdn.msaied.com/01M22N44A70A5MC2S599JP0MPH.webp) [Mohamed Said](https://msaied.com/public#person) Published 10 Jul 2026 · Updated 10 Jul 2026 · 3 min read ShareCopy linkCopied ![Eloquent Custom Relations: Building a HasManyThrough Alternative for Complex Joins](https://cdn.msaied.com/405/9567c07e9cdd10de853dc989784f6c5c.png) On this page +1. [Why Built-In Relations Sometimes Fall Short](#why-built-in-relations-sometimes-fall-short) 2. [Anatomy of an Eloquent Relation](#anatomy-of-an-eloquent-relation) 3. [A Concrete Example: HasManyByCode](#a-concrete-example-hasmanybycode) 4. [Wiring It Into the Model](#wiring-it-into-the-model) 5. [Handling withCount and whereHas](#handling-codewithcountcode-and-codewherehascode) 6. [Key Takeaways](#key-takeaways) Why Built-In Relations Sometimes Fall Short ------------------------------------------- Laravel ships with `HasMany`, `BelongsToMany`, `HasManyThrough`, and friends. They handle 95% of real-world schemas. But occasionally you hit a domain shape that none of them model cleanly — for example, a polymorphic pivot with an extra filtering dimension, or a relation that spans a non-FK column like a `code` string shared across two tables. The answer is not a raw query shoved into an accessor. The answer is a **proper Eloquent relation class** that participates in eager loading, `with()`, `whereHas()`, and `withCount()`. Anatomy of an Eloquent Relation ------------------------------- Every relation extends `Illuminate\Database\Eloquent\Relations\Relation`. The three methods you must implement are: ```php public function addConstraints(): void; public function addEagerConstraints(array $models): void; public function match(array $models, Collection $results, string $relation): array; public function getResults(): mixed; ``` `addConstraints` applies the WHERE clause for a single-model load. `addEagerConstraints` receives the full parent batch and applies an `IN` clause. `match` stitches results back onto each parent model. A Concrete Example: HasManyByCode --------------------------------- Imagine `Campaign` has a `tracking_code` column, and `Conversion` also has `tracking_code` — but there is no FK. A `HasManyByCode` relation solves this cleanly. ```php namespace App\Relations; use Illuminate\Database\Eloquent\Collection; use Illuminate\Database\Eloquent\Model; use Illuminate\Database\Eloquent\Relations\Relation; class HasManyByCode extends Relation { public function __construct( protected string $foreignKey, protected string $localKey, Model $related, Model $parent, ) { parent::__construct($related->newQuery(), $parent); } public function addConstraints(): void { if (static::$constraints) { $this->query->where( $this->foreignKey, $this->parent->{$this->localKey} ); } } public function addEagerConstraints(array $models): void { $keys = collect($models) ->pluck($this->localKey) ->filter() ->unique() ->values(); $this->query->whereIn($this->foreignKey, $keys); } public function initRelation(array $models, string $relation): array { foreach ($models as $model) { $model->setRelation($relation, $this->related->newCollection()); } return $models; } public function match(array $models, Collection $results, string $relation): array { $dictionary = $results->groupBy($this->foreignKey); foreach ($models as $model) { $key = $model->{$this->localKey}; $model->setRelation( $relation, $dictionary->get($key, $this->related->newCollection()) ); } return $models; } public function getResults(): mixed { return $this->query->get(); } } ``` Wiring It Into the Model ------------------------ Add a convenience method on `Campaign` that mirrors how built-in relations are declared: ```php use App\Relations\HasManyByCode; use App\Models\Conversion; class Campaign extends Model { public function conversions(): HasManyByCode { return new HasManyByCode( foreignKey: 'tracking_code', localKey: 'tracking_code', related: new Conversion(), parent: $this, ); } } ``` Now standard Eloquent APIs work without modification: ```php // Eager load — single query with whereIn $campaigns = Campaign::with('conversions')->get(); // Existence check $active = Campaign::whereHas('conversions', fn ($q) => $q->where('converted_at', '>=', now()->subDays(7)) )->get(); // Count $campaigns = Campaign::withCount('conversions')->get(); ``` Handling `withCount` and `whereHas` ----------------------------------- These features rely on `getRelationExistenceQuery()`. The default implementation works for simple cases, but if your foreign key lives on the related table you may need to override it: ```php public function getRelationExistenceQuery( Builder $relatedQuery, Builder $parentQuery, mixed $columns = ['*'] ): Builder { return $relatedQuery ->select($columns) ->whereColumn( $this->foreignKey, $parentQuery->qualifyColumn($this->localKey) ); } ``` This single override unlocks `whereHas`, `doesntHave`, and `withCount` for your custom relation. Key Takeaways ------------- - Extend `Relation` directly; implement `addConstraints`, `addEagerConstraints`, `match`, and `getResults` as a minimum. - `initRelation` prevents `null` relation errors on models that have no matching rows. - Override `getRelationExistenceQuery` to unlock `whereHas` and `withCount`. - Group results by the foreign key in `match` — `groupBy` on a Collection is O(n) and avoids nested loops. - Custom relations are first-class citizens: they work with `load()`, `loadMissing()`, and API resources. - [laravel](https://msaied.com/public/articles?search=laravel) - [eloquent](https://msaied.com/public/articles?search=eloquent) - [database](https://msaied.com/public/articles?search=database) - [orm](https://msaied.com/public/articles?search=orm) Frequently asked questions --------------------------- Can a custom relation class be used with Laravel's API resources and `whenLoaded()`?Yes. As long as `setRelation()` is called in `match()` and `initRelation()` seeds an empty collection, `$model->relationLoaded('conversions')` returns true after eager loading, and `whenLoaded()` in a resource works identically to built-in relations. Does this approach work with `loadMissing()` on already-retrieved models?It does. `loadMissing()` calls `addEagerConstraints` on the batch of models that lack the relation, then calls `match()` to hydrate them — the same code path as `with()` during the initial query. How do I add default ordering to the custom relation?Apply `$this->query->orderBy(...)` inside `addConstraints()`. For eager loads, ordering is applied per-model after `match()` groups results, so you may also sort the collection inside `match()` before calling `setRelation()`. ![Mohamed Said](https://cdn.msaied.com/01M22N44A70A5MC2S599JP0MPH.webp)About the author ---------------- [Mohamed Said](https://msaied.com/public#person)Senior Backend Engineer specializing in Laravel, scalable SaaS platforms, APIs, and cloud infrastructure. I build secure, high-performance web applications that help businesses grow. [About](https://msaied.com/public#about) [GitHub ↗](https://github.com/EG-Mohamed) [LinkedIn ↗](https://www.linkedin.com/in/msaiedm/) [WhatsApp ↗](https://wa.me/201094619204) [Email Address ↗](mailto:hello@msaied.com) [My CV ↗](https://drive.google.com/file/u/0/d/1MF20IPRJyzfy32mhEutjL5EpSls0w2Q8/view) [Previous articleLaravel 13: New Features, Helpers, and Practical Upgrade Notes](https://msaied.com/public/articles/laravel-13-new-features-helpers-and-practical-upgrade-notes-1) [Next articlePasswordless Sign-In with Fortify Two-Factor Support in Laravel](https://msaied.com/public/articles/passwordless-sign-in-with-fortify-two-factor-support-in-laravel) On this page ------------- 1. [Why Built-In Relations Sometimes Fall Short](#why-built-in-relations-sometimes-fall-short) 2. [Anatomy of an Eloquent Relation](#anatomy-of-an-eloquent-relation) 3. [A Concrete Example: HasManyByCode](#a-concrete-example-hasmanybycode) 4. [Wiring It Into the Model](#wiring-it-into-the-model) 5. [Handling withCount and whereHas](#handling-codewithcountcode-and-codewherehascode) 6. [Key Takeaways](#key-takeaways) ### Have a technical challenge? Tell me what you’re building. I reply within two working days. 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