example.php

<?php

namespace App\Models;

use App\Models\Traits\HasScopedRelations;
use Illuminate\Database\Eloquent\Model;

class User extends Model
{
    use HasScopedRelations;

    public function addresses()
    {
        return $this->hasMany('App\Models\Address');
    }

    public function primaryAddress()
    {
        return $this->hasOneScoped('App\Models\Address')->with(['type' => 'primary']);
    }
}

HasOneScoped.php

<?php

namespace App\Models\Relations;

use Illuminate\Database\Eloquent\Relations\HasOne;

class HasOneScoped extends HasOne
{
    protected $attributes = [];

    public function addConstraints()
    {
        parent::addConstraints();

        if(static::$constraints){
            $this->query->where($this->attributes);
        }
    }

    public function addEagerConstraints(array $models)
    {
        parent::addEagerConstraints($models);
        $this->query->where($this->attributes);
    }

    public function create(array $attributes)
    {
        return parent::create(array_merge($attributes, $this->attributes));
    }

    /**
     * Add attribute scope.
     *
     * @param $attributes
     *
     * @return $this
     */
    public function with($attributes)
    {
        $this->attributes = $attributes;

        $this->query->where($attributes);

        return $this;
    }
}

HasScopedRelation.php

<?php

namespace App\Models\Traits;

use App\Models\Relations\HasOneScoped;

/**
 * Trait HasScopedRelations can be added to an eloquent model to extend functionality
 */
trait HasScopedRelations
{

    /**
     * @param $related
     * @param null $foreignKey
     * @param null $localKey
     *
     * @return HasOneScoped
     */
    public function hasOneScoped($related, $foreignKey = null, $localKey = null)
    {
        $foreignKey = $foreignKey ?: $this->getForeignKey();

        $instance = new $related;

        $localKey = $localKey ?: $this->getKeyName();

        return new HasOneScoped($instance->newQuery(), $this, $instance->getTable().'.'.$foreignKey, $localKey);
    }

}

@antwaanvh