外部数据库中的Laravel外键引用表？

Question

I'm new to Laravel and databases in general. I'm writing a web application for student evaluations. I have an existing MySQL database that contains everything I need already; however, I am using Laravel's auth user table and trying to add a foreign key that references a Teacher table in the MySQL database. I keep getting the following error...

php artisan migrate:fresh

In Connection.php line 664:
SQLSTATE[HY000]: General error: 1215 Cannot add foreign key constraint (SQL: alter table users add constraint users_teacher_id_foreign foreign key (teacher_id) references p4j.teacher (teacher_id))

In Connection.php line 458: SQLSTATE[HY000]: General error: 1215 Cannot add foreign key constraint

My code is as follows...

config/database.php

'mysql' => [
    'driver' => 'mysql',
    'host' => env('DB_HOST', '127.0.0.1'),
    'port' => env('DP_PORT', '3306'),
    'database' => env('DB_DATABASE', 'p4j'),
    'username' => env('DB_USERNAME', 'root'),
    'password' => env('DP_PASSWORD', '**mypassword**'),
    'charset' => 'utf8',
    'collation' => 'utf8_unicode_ci',
    'prefix' => '',
],

database/migrations/2014_10_12_000000_create_users_table.php

public function up()
{
    Schema::create('users', function (Blueprint $table) {
        $table->increments('id');
        $table->string('name');
        $table->string('email')->unique();
        $table->string('password');
        $table->integer('teacher_id')->unsigned();
        $table->rememberToken();
        $table->timestamps();
    });

    Schema::table('users', function (Blueprint $table) {
          $table->foreign('teacher_id')->references('teacher_id')->on('p4j.teacher');
    });
}

app/User.php

protected $fillable = [
    'name', 'email', 'password', 'teacher_id'
];

/**
 * The attributes that should be hidden for arrays.
 *
 * @var array
 */
protected $hidden = [
    'password', 'remember_token',
];

Answer 1

Couple possible issues and solutions are that your db engine is not INNODB or loading order of migration files is incorrect. For the first solution you need to add $table->engine = 'InnoDB'; inside of Schema::create and if this doesn't fix the issue, i'll need you to post the names of your users and teacher migrations

Possible fix:

Both PRIMARY KEY constraint and UNIQUE constraint uses to enforce Entity integrity (defines a row as a unique entity for a particular table), but primary keys do not allow null values. Specifies the column that uniquely identifies a row in the table. The identified columns must be defined as NOT NULL.

And since laravel increment function also

Auto-incrementing UNSIGNED INTEGER (primary key) equivalent column.

Simply removing ->default(null); and rerunning migrations should potentially fix the problem.