Question about Laravel's foreign keys implementation

Question

I'm new at Laravel. I cannot understand how to insert data into the foreign key. Thank you for any answer. There are my code fragments:

name="user_id" is the foreign key of the Users' table which is held in the Course's table so I want that by the firstname and lastname attributes of the Users' table the Courses table would get user's ID.

<div class="row d-flex justify-content-center">
                            <div class="col-md-4">
                            <div class="form-group">
                                <select class="form-control dropdown-menu-arrow" name="subject" required>
                                    <option value="" selected disabled>Kurso vadovas</option>
                                    @foreach ($users as $user)
                                <option name="user_id">{{$user->firstname}} {{$user->lastname}}</option>
                                @endforeach
                                </select>
                            </div>
                        </div> 



 Schema::create('courses', function (Blueprint $table) {
            $table->string('course_title');
            $table->string('subject');
            $table->mediumText('comments');
            $table->bigIncrements('course_id');
            $table->mediumText('description');
            $table->integer('user_id');
            $table->foreign('user_id')->references('id')->on('users');
            $table->timestamps();
        });
 Schema::create('users', function (Blueprint $table) {
            $table->bigIncrements('id');
            $table->string('firstname');
            $table->string('lastname');
            $table->string('usertype');
            $table->string('email')->unique();
            $table->timestamp('email_verified_at')->nullable();
            $table->string('password');
            $table->rememberToken();
            $table->timestamps();
        });

Answer 1

请测试这个

<option name="{{$user->id}}">{{$user->firstname}} {{$user->lastname}}</option>

Answer 2

you can use like this:

<select class="form-control dropdown-menu-arrow" name="subject" required>
   <option value="" selected disabled>Kurso vadovas</option>
  @foreach ($users as $user)
     <option value="{{$user->id}}">{{$user->firstname}} {{$user->lastname}}</option>
 @endforeach
 </select>

and then get user id from controller:

$user_id = $request->subject;