{
"users":[
    {
        "id":1,
        "fields":[
            {
                "id":2,
                "name":"first_name",
                "display_name":"Имя",
                "pivot":{
                    "user_id":1,
                    "field_id":2,
                    "value":"Вася"
                }
            },
            {
                "id":2,
                "name":"last_name",
                "display_name":"Фамилия",
                "pivot":{
                    "user_id":1,
                    "field_id":2,
                    "value":"Серый"
                }
            }
        ]
    }
]

{
"users":[
    {
        "id":1,
        "first_name": "Вася",
        "last_name": "Серый",
        "fields":[
            {
                "id":2,
                "name":"first_name",
                "display_name":"Имя",
                "pivot":{
                    "user_id":1,
                    "field_id":2,
                    "value":"Вася"
                }
            },
            {
                "id":2,
                "name":"last_name",
                "display_name":"Фамилия",
                "pivot":{
                    "user_id":1,
                    "field_id":2,
                    "value":"Серый"
                }
            }
        ]
    }
]

}

public function fields()
{
    return $this->morphToMany('AppModelsField', 'pivot')->withPivot('value');
}

protected $appends = ['last_name'];

public function getLastNameAttribute()
{
    return $this->fields->where('field_id', 2)->first()->pivot->value;
}