Understanding Laravel Eloquent Relationships: HasManyThrough, Polymorphic, and Advanced Query Techniques

Laravel's Eloquent ORM provides powerful relationship types that simplify complex data retrieval. The "remote one‑to‑many" (hasManyThrough) relationship lets a Country model access all related Post records through the intermediate User model, requiring specific foreign keys in the countries , users , and posts tables.

countries
    id - integer
    name - string

users
    id - integer
    country_id - integer
    name - string

posts
    id - integer
    user_id - integer
    title - string

In the Country model, the posts() method returns $this->hasManyThrough('App\Post', 'App\User') . Optional third and fourth parameters allow custom foreign keys, while the fifth and sixth parameters set local keys.

<?php
namespace App;
use Illuminate\Database\Eloquent\Model;

class Country extends Model {
    /**
     * Get all posts for a given country.
     */
    public function posts() {
        return $this->hasManyThrough('App\Post', 'App\User');
    }
}

Polymorphic relationships enable a single model to belong to multiple other models. The comments table stores commentable_id and commentable_type to reference either Post or Video . The Comment model defines a commentable() method using morphTo() , while Post and Video define comments() with morphMany() .

<?php
namespace App;
use Illuminate\Database\Eloquent\Model;

class Comment extends Model {
    public function commentable() {
        return $this->morphTo();
    }
}

class Post extends Model {
    public function comments() {
        return $this->morphMany('App\Comment', 'commentable');
    }
}

class Video extends Model {
    public function comments() {
        return $this->morphMany('App\Comment', 'commentable');
    }
}

Custom polymorphic type mappings can be defined with Relation::morphMap() in a service provider, allowing you to store simple strings instead of fully‑qualified class names.

use Illuminate\Database\Eloquent\Relations\Relation;

Relation::morphMap([
    'posts' => 'App\Post',
    'videos' => 'App\Video',
]);

Many‑to‑many polymorphic relationships let models like Post and Video share a Tag model via a taggables pivot table. Each model defines a tags() method using morphToMany() , and the Tag model defines posts() and videos() with morphedByMany() .

<?php
namespace App;
use Illuminate\Database\Eloquent\Model;

class Post extends Model {
    public function tags() {
        return $this->morphToMany('App\Tag', 'taggable');
    }
}

class Video extends Model {
    public function tags() {
        return $this->morphToMany('App\Tag', 'taggable');
    }
}

class Tag extends Model {
    public function posts() {
        return $this->morphedByMany('App\Post', 'taggable');
    }
    public function videos() {
        return $this->morphedByMany('App\Video', 'taggable');
    }
}

Retrieving related data can be done via dynamic properties (e.g., $post->comments ) or by calling the relationship method to obtain a query builder for further constraints. Laravel also offers methods such as has() , whereHas() , doesntHave() , and withCount() to filter models based on the existence or count of related records.

// Example: Get posts with at least one comment
$posts = App\Post::has('comments')->get();

// Example: Get posts with three or more comments
$posts = App\Post::has('comments', '>=', 3)->get();

// Example: Count comments for each post
$posts = App\Post::withCount('comments')->get();

These techniques enable developers to write expressive, efficient queries while keeping model definitions clean and maintainable.