Get the primary key from table using belongsTo

I hav a following class code:

Student

<?php

namespace App\Models;

use Illuminate\Database\Eloquent\Factories\HasFactory;
use Illuminate\Database\Eloquent\Model;

class Student extends Model
{
    use HasFactory;  
    protected $fillable = [
        'name',
        'email',
        'academic_registration',
        'created_at',
        'updated_at'
    ];
    protected $table = 'students';    
}

StudentDisciplineClassroom (The Pivot table)

<?php

namespace App\Models;

use Illuminate\Database\Eloquent\Factories\HasFactory;
use Illuminate\Database\Eloquent\Model;
use Illuminate\Database\Eloquent\Relations\BelongsTo;

class StudentDisciplineClassroom extends Model
{
    use HasFactory;
    protected $fillable = [
        'student',
        'ra',
        'discipline',
        'classroom',
        'created_at',
        'updated_at'
    ];
    protected $table = 'students_disciplines_classrooms';     

    public function studentRelation(): BelongsTo
    {
        return $this->belongsTo(Student::class, 'ra', 'academic_registration');
    }          
	
	...

}

I need to do this works

public function studentRelation(): BelongsTo {

   return $this->belongsTo(Student::class, 'ra', 'academic_registration');

}          

The problem:

'academic_registration' in Student::class is not the primary key of table and i need get the prymary key (id) this

Then i tryed

$all = $sdc::select('ra')->with('studentRelation')->where('classroom',$request->classroom)->distinct('student')->get();

but studentRelation just returns null