一般错误：1215 无法在 Laravel 中添加外键约束

Question

做迁移我得到这个错误： SQLSTATE[HY000]: General error: 1215 Cannot add foreign key constraint (SQL: alter table booksadd constraint books_writer_id_foreignforeign key ( writer_id) references writers( id)) 我已经尝试了很多事情，但没有人看起来工作.

2018_02_18_3165165_create_books_table.php

<?php

use Illuminate\Support\Facades\Schema;
use Illuminate\Database\Schema\Blueprint;
use Illuminate\Database\Migrations\Migration;

class CreateBooksTable extends Migration
{
    /**
     * Run the migrations.
     *
     * @return void
     */
    public function up()
    {
        Schema::create('books', function (Blueprint $table) {
            $table->engine = 'InnoDB';

            $table->increments('id');


            $table->string('name');
            $table->text('description');

            $table->integer('numPages');
            $table->enum('language', ['spanish', 'english']);

            $table->date('wrote_date')->nullable();


            $table->timestamp('created_at')->useCurrent();
            $table->timestamp('updated_at')->default(DB::raw('CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP'));
        });
        Schema::table('books', function (Blueprint $table) {
            $table->integer('writer_id')->unsigned();
            $table->foreign('writer_id')->references('id')->on('writers');
        });
    }

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

2018_02_18_192915_create_writers_table

use Illuminate\Support\Facades\Schema;
use Illuminate\Database\Schema\Blueprint;
use Illuminate\Database\Migrations\Migration;

class CreateWritersTable extends Migration
{
    /**
     * Run the migrations.
     *
     * @return void
     */
    public function up()
    {
        Schema::create('writers', function (Blueprint $table) {
            $table->increments('id');
            $table->string('name');
            $table->text('description');

            $table->string('nationality');
            $table->date('year_date')->nullable();
            $table->date('dead_date')->nullable();

            $table->timestamp('created_at')->useCurrent();
            $table->timestamp('updated_at')->default(DB::raw('CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP'));
        });
    }

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

编辑：错误是因为第一次迁移是书籍，然后是导致错误的作者。

Answer 1

有时根据我的经验，太接近的时间戳会破坏代码并抛出异常，因为程序认为在书籍表尝试更改 writers_table 时间戳之后创建了 writers 表，例如：
2018_02_16_31615

Answer 2

unsignedInteger因为你正在使用它increments()

$table->unsignedInteger('writer_id')->nullable();