php – 刷新迁移时的外键约束错误 – Laravel

因为我对Laravel相当新,所以我无法理解.

我想将一个外键添加到现有的表中,这样可行,但是当我刷新迁移时,我收到此错误：

[IlluminateDatabaseQueryException]
SQLSTATE[23000]: Integrity constraint violation: 1217 Cannot delete or update a parent row: a foreign key
constraint fails (SQL: drop table `battles`)

[PDOException]
SQLSTATE[23000]: Integrity constraint violation: 1217 Cannot delete or update a parent row: a foreign key
constraint fails

这些是我目前的迁移：

表项目

class CreateProjectsTable extends Migration
{
    /**
     * Run the migrations.
     *
     * @return void
     */
    public function up()
    {
        Schema::create('projects',function (Blueprint $table) {
            $table->increments('id');
            $table->string('title');
            $table->string('body');
            $table->string('tags');
            $table->string('img');
            $table->string('img_tricolor');
            $table->integer('user_id')->unsigned();
            $table->foreign('user_id')->references('id')->on('users')->onDelete('cascade');
            $table->timestamps();
        });
    }

    /**
     * Reverse the migrations.
     *
     * @return void
     */
    public function down()
    {
        Schema::drop('projects');
    }
}

表战

class CreateBattlesTable extends Migration
{
    /**
     * Run the migrations.
     *
     * @return void
     */
    public function up()
    {
        Schema::create('battles',function (Blueprint $table) {
            $table->increments('id');
            $table->string('battle_theme');
            $table->boolean('battle_active');
        });
    }

    /**
     * Reverse the migrations.
     *
     * @return void
     */
    public function down()
    {
        Schema::drop('battles');
    }
}

为项目中的战斗添加外键

class AddProjectsBattleIdFk extends Migration
{
    /**
     * Run the migrations.
     *
     * @return void
     */
    public function up()
    {
        Schema::table('projects',function (Blueprint $table) {
            $table->integer('battle_id')->unsigned();
            $table->foreign('battle_id')->references('id')->on('battles')->onDelete('set null');
        });
    }

    /**
     * Reverse the migrations.
     *
     * @return void
     */
    public function down()
    {
        Schema::table('projects',function (Blueprint $table) {
            //
        });
    }
}

我想这与战斗表有关.