General error 1215: cannot add foreign key constraint

Question

I'm getting the following error:

SQLSTATE[HY000]: General error: 1215 Cannot add foreign key constraint (SQL: alter table subtags add constraint subtags_tag_id_foreign foreign key (tag_id) references id (tags) on delete cascade)

After looking at other answers regarding this error on stackoverflow and this blogpost: https://www.percona.com/blog/2017/04/06/dealing-mysql-error-code-1215-cannot-add-foreign-key-constraint/ I still can't figure out why I'm getting the error.

This is the migration for the subtag table:

public function up()
{
    Schema::create('subtags', function (Blueprint $table) {
        $table->engine = 'InnoDB';
        $table->integer('tag_id')->unsigned();
        $table->integer('id')->unsigned();
        $table->string('name');
        $table->timestamps();
    });

    Schema::table('subtags', function (Blueprint $table) {
        $table->foreign('tag_id')
            ->references('tags')
            ->on('id')
            ->onDelete('cascade');

        $table->primary(array('tag_id', 'id'));
    });
}

At the point of error the tag table has already been created and I can't spot any typo's but just to make sure, here is the tag table migration:

public function up()
{
    Schema::create('tags', function (Blueprint $table) {
        $table->engine = 'InnoDB';
        $table->increments('id');
        $table->string('name');
        $table->timestamps();
    });
}

Looking at the query, it looks like you have the foreign table and columns backwards, so it should be `->on('tags')->references('id')` — Niet the Dark Absol, Jun 09 '18 at 14:32
@NiettheDarkAbsol Yep, There is the problem.... well, that is a good hour of my life wasted xD. Thanks a lot! — Jjampong, Jun 09 '18 at 14:37
Sorry to hear you wasted an hour of your life. Next time, you might reduce the wasted time to 5 minutes instead by reading documentation like: https://laravel.com/docs/5.6/migrations#foreign-key-constraints — Bill Karwin, Jun 09 '18 at 15:45

score 1 · Accepted Answer · answered Jun 09 '18 at 15:13

Your query shows that you have misplaced the column & table in:

Schema::table('subtags', function (Blueprint $table) {
    $table->foreign('tag_id')
          ->references('tags')
          ->on('id')
          ->onDelete('cascade');

    $table->primary(array('tag_id', 'id'));
});

Reference should be "column" on "Table" like:

Schema::table('subtags', function (Blueprint $table) {
    $table->foreign(id'tag_id')
          ->references('id')
          ->on('tags')
          ->onDelete('cascade');

    $table->primary(array('tag_id', 'id'));
});

score 0 · Answer 2 · answered Jun 10 '18 at 08:38

0

You can disable check foreigns keys before migrate

DB::statement('SET FOREIGN_KEY_CHECKS=0;');
...
DB::statement('SET FOREIGN_KEY_CHECKS=1;');

answered Jun 10 '18 at 08:38

webdevtr

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General error 1215: cannot add foreign key constraint

2 Answers2