How to make a no primary key with datatype sting as foreign key in another table

Question

I am trying to make a non primary key as foreign key in another table whose datatype is string.I'm trying to reference doctor_id from users table

Referenced Table

Referencing Table

Answer 1

you have to use unique modifier on your column.

$table->string('doctor_id')->unique();

however, if you set nullable as the modifier like:

$table->string('doctor_id')->unique()->nullable();

it will present another problem when it has multiple records with the same value of null .