Laravel migration添加foreign key时提示错误：key column ‘xxx’ dons’t exist的原因和解决办法

Laravel migration添加foreign key时报错，错误信息为：key column ‘xxx’ dons’t exist。错误代码如下：

Schema::create('emperors', function (Blueprint $table) {
  $table->bigIncrements('id');
  $table->string('name');
  $table->foreign('dynasty_id')->references('id')->on('dynasties')->onDelete('cascade');
  $table->timestamps();
});

上面的代码在添加外键dynasty_id的时候报错，其实这是一个新手错误，没认真看文档，实际上在创建外键关系时，首先要创建该列，正确代码：

Schema::create('emperors', function (Blueprint $table) {
  $table->bigIncrements('id');
  $table->string('name');
  $table->unsignedInteger('dynasty_id')->nullable();
  $table->foreign('dynasty_id')->references('id')->on('dynasties')->onDelete('cascade');
  $table->timestamps();
});