mysql foreign key error in Laravel 5.1

Question

I have created two migrations:

Migration 1

    Schema::create('responders', function (Blueprint $table) {
        $table->increments('id');
        $table->string('user_id');
        $table->double('latitude', 10, 6);
        $table->double('longitude', 10, 6);
        $table->timestamps();
    });

Migration 2

    Schema::create('devices', function (Blueprint $table) {
        $table->increments('id');
        $table->string('user_id');
        $table->string('device_id');
        $table->string('device_token');
        $table->timestamps();

        $table->foreign('user_id')
            ->references('user_id')
            ->on('responders')
            ->onDelete('cascade');

    });

When I start migration the error message is being thrown:

[Illuminate\Database\QueryException]
SQLSTATE[HY000]: General error: 1215 Cannot add foreign key constraint (SQL: alter table devices add constraint devices_user_id_foreign foreign key (user_id) references responders (user_id) on delete cascade)
[PDOException]
SQLSTATE[HY000]: General error: 1215 Cannot add foreign key constraint

I specifically took care that the data types are the same. In this case both are of type string. Why does the foreign key can't be established?

The foreign key can't be defined because there is no unique index on the user_id column of the responders table. If you want to define a foreign key, add an index on the referenced column(s). — spencer7593
are you sure $table->string('user_id'); should be string not integer? — maytham-ɯɐɥʇʎɐɯ
there is some thing also wrong with your logic I think, let me understand so I can help you, responders will have many devices is that correct? and responders belong to a user? is that correct — maytham-ɯɐɥʇʎɐɯ

ARIF MAHMUD RANA ARIF MAHMUD RANA · Accepted Answer · 2015-11-01T05:42:06

A foreign key is a column (or columns) that references a column (most often the primary key) of another table. The purpose of the foreign key is to ensure referential integrity of the data. In other words, only values that are supposed to appear in the database are permitted.

Here user_id in responders table isn't a key that's why it's showing this error.

mysql foreign key error in Laravel 5.1

1 Answers