我使用的是mysql v5.7和laravelv5.6，我在产品表中添加了外键。
但这样的错误。

SQLSTATE[HY000]: General error: 1215 Cannot add foreign key constraint (SQL: alter table `product` add constraint `product_category_id_foreign` foreign key (`category_id`) references `category` (`id`) on delete cascade)

这是我的产品表

public function up()
    {
        Schema::create('product', function (Blueprint $table) {
            $table->increments('id');
            $table->integer('category_id')->unsigned()->nullable();
            $table->integer('admin_id')->unsigned()->nullable();
            $table->string('name');
            $table->string('description');
            $table->integer('price');
            $table->binary('image')->nullable(false)->change();
            $table->timestamps();
        });
        Schema::table('product', function($table){
            $table->foreign('category_id')->references('id')->on('category')->onDelete('cascade');
            $table->foreign('admin_id')->references('id')->on('admin')->onDelete('cascade');
        });
    }

这是我的分类表

public function up()
    {
        Schema::create('category', function (Blueprint $table) {
            $table->increments('id');
            $table->string('title');
            $table->timestamps();
        });
    }

这里怎么了？