Make eloquent model for subtype - php

Question

I'm creating a school platform where students, teachers,... can login using their credentials. To reduce duplicate data I did not make a separate table called students, instead I keep all the data in the users table.
To know if a user is a student I a have a table that is called enrolments, in this table a user_id , schoolyear_id and class_id is stored.
I already made a student model that refers to the users table, but how can I ensure that this model only passes students?
EER:
Student.php:
<?php
namespace App;
class Student extends User
{
protected $table= 'users';
public function enrollments(){
return $this->belongsToMany(Enrollment::class);
}
}
User.php:
<?php
namespace App;
use Illuminate\Notifications\Notifiable;
use Illuminate\Foundation\Auth\User as Authenticatable;
use Spatie\Permission\Traits\HasRoles;
use Illuminate\Support\Facades\Auth;
class User extends Authenticatable
{
use Notifiable;
use HasRoles;
/**
* The attributes that are mass assignable.
*
* #var array
*/
protected $fillable = [
'first_name','last_name', 'password'
];
/**
* The attributes that should be hidden for arrays.
*
* #var array
*/
protected $hidden = [
'password', 'remember_token',
];
public function profiles(){
return $this->hasOne(Profile::class);
}
}
What I want to achieve is that when I call the Student::all(); function I get all the users who are enrolled in the school,hence students.

Answer 1

Check out model events: https://laravel.com/docs/5.5/eloquent#events
You should be able to drop this into your student model for a test:
protected static function boot(){
parent::boot();
static::retrieved(function($thisModel){
if($thisModel->isNotAStudent or whatever logic you need){
return false;
}
}
}
I'm still on 5.4, which does not have the retrieved model event built in, but returning false generally stops the call from going through. So applying that logic to the retrieved event may stop that model instance from being returned if it is not a student, but allow students to be returned. Just a thought.

Answer 2

Your provided solution lead me in the right direction. My problem is solved by using global scope:
<?php
namespace App;
use Illuminate\Database\Eloquent\Builder;
use Illuminate\Support\Facades\DB;
class Student extends User
{
protected $table= 'users';
protected static function boot()
{
parent::boot();
static::addGlobalScope('student', function (Builder $builder) {
$builder->whereExists(function ($query) {
$query->select(DB::raw(1))
->from('enrollments')
->whereRaw('enrollments.user_id = users.id');
});
});
}
public function enrollments(){
return $this->belongsToMany(Enrollment::class);
}
}