Laravel model trait for easily adding self-referential tables

IsSelfReferencing.php

<?php

namespace App;

use Illuminate\Database\Eloquent\Builder;

trait IsSelfReferencing
{
    /**
     * The self referencing key on the database table.
     *
     * @var string
     */
    protected $referenceKey = 'parent_id';

    /**
     * The belongsTo parent relationship.
     *
     * @return \Illuminate\Database\Eloquent\Relations\BelongsTo
     */
    public function parent()
    {
        return $this->belongsTo(static::class, $this->referenceKey);
    }

    /**
     * The hasMany children relationship.
     *
     * @return \Illuminate\Database\Eloquent\Relations\HasMany
     */
    public function children()
    {
        return $this->hasMany(static::class, $this->referenceKey);
    }

    /**
     * The hasMany descendants relationship.
     *
     * @return \Illuminate\Database\Eloquent\Relations\HasMany
     */
    public function descendants()
    {
        return $this->children()->with('descendants');
    }

    /**
     * Determine if the record has no parent.
     *
     * @return bool
     */
    public function isRoot()
    {
        return is_null($this->{$this->referenceKey});
    }

    /**
     * Determine if the record is a child of another.
     *
     * @return bool
     */
    public function isChild()
    {
        return !$this->isRoot();
    }

    /**
     * Scope the query for root entries only.
     *
     * @param Builder $query
     *
     * @return Builder
     */
    public function scopeRoots(Builder $query)
    {
        return $query->whereNull($this->referenceKey);
    }
}

Glad I could help you @q10242! 😄

Is there any way to get the siblings of a model ?

public function siblings()
{
    return $this->hasMany(static::class, $this->referenceKey, $this->referenceKey);
}

The above code works when the foreign key value is not NULL, but fails essentially for parent categories.

@pravnkay I think you'll have to return differently if the model you're requesting doesn't have a parent_id:

public function siblings()
{
    return $this->isRoot() 
        ? $this->roots()->whereKeyNot($this->{$this->referenceKey})
        : $this->hasMany(static::class, $this->referenceKey, $this->referenceKey);
}