将外键添加到表中

Question

我试图向用户表中添加一个permission_id外键，该外键将引用权限表中的id。目前，我在移民之后建造了：

<?php
class AddPermissionIdForeignKeyToUsersTable extends Migration
{
    public function up()
    {
        Schema::table('users', function (Blueprint $table) {
            $table->unsignedInteger('permission_id')->after('id');
            $table->foreign('permission_id')->references('id')->on('permissions')->onDelete('cascade');
        });
    }

    public function down()
    {
        Schema::table('users', function (Blueprint $table) {
            $table->dropColumn('permission_id');
        });
    }
}

但我得到了以下错误：

SQLSTATE23000:完整性约束违反: 1452不能添加或更新子行:外键约束失败(eyes.#sql-22f0_3dd、约束users_permission_id_foreign外键(permission_id)在删除级联中引用p ermissions (id) ) (SQL: alter users添加约束users_permission_id_foreign外键(permission_id)引用permissions (id) 在PDOStatement.php第144行： SQLSTATE23000:完整性约束违反:1452年不能添加或更新子行:外键约束失败(eyes.#sql-22f0_3dd、constraint users_permission_id_foreign Integrity (permission_id)引用p ermissions (id) ) 在PDOStatement.php第142行中： SQLSTATE23000:完整性约束违反:1452年不能添加或更新子行:外键约束失败(eyes.#sql-22f0_3dd、constraint users_permission_id_foreign Integrity (permission_id)引用p ermissions (id) )

Answer 1

如果表用户中已经有数据，则需要将字段permission_id声明为可空。

$table->unsignedInteger('permission_id')->nullable()->after('id');