Integrity constraint violation laravel 9

Question

I am using Laravel 9 to set up my migrations but I keep getting this error Integrity constraint violation: 1452 Cannot add or update a child row: a foreign key constraint fails I have looked this up and I'm very confused as my user_id in table urls does reference the id column on the users table?

I have also ensured that I have protected fillable for the user_id in my url.php model which I saw as an answer to a similar question.

public function up()
    {
        Schema::create('urls', function (Blueprint $table) {
            $table->id();
            $table->text('full_url');
            $table->string('short_url')->unique();
            $table->foreignId('user_id')->constrained();
            $table->timestamps();
        });
    }

    public function up()
    {
        Schema::create('users', function (Blueprint $table) {
            $table->id();
            $table->string('name');
            $table->string('email')->unique();
            $table->timestamp('email_verified_at')->nullable();
            $table->string('password');
            $table->rememberToken();
            $table->timestamps();
        });
    }

Answer 1

Foreign key constraints are one of the mechanisms used to ensure data integrity. In this case, it means that the foreign key you're about to insert must have a correspondence within the associated foreign table.

Let me give an example. Let's say you have a posts table that has a user_id foreign key. Then in your users table, you have the following ids: 1, 2, 3 and 4. If you try to insert/update a post, the user_id provided must exist within the users table. So that means that if you tried to insert a new post that has a user_id of 7, it would fail because there's no id of 7 in the users table.