How to make a foreign key not using primary key

php laravel foreign-keys relation laravel-migrations

PHP Snippet 1:

Schema::table('pasien', function (Blueprint $table) {
   $table->unique('RM_id');
});

PHP Snippet 2:

public function up()
{
    Schema::create('pasien', function (Blueprint $table) {
        $table->increments('id');
        $table->string('No_RM')->unique();
        $table->timestamps();
    });
}

PHP Snippet 3:

public function up()
{
    Schema::create('data_primary', function (Blueprint $table) {
        $table->increments('id');
        $table->string('RM_id');
        $table->string('file_primary');
        $table->timestamps();

        $table->foreign('RM_id')->references('RM_id')->on('pasien')
                ->onUpdate('cascade')->onDelete('cascade');
    });
}