Laravel 5 迁移：重命名列时出错

Question

我是 Laravel 的新手并且有这样的迁移：

public function up()
{
    Schema::table('mytable', function(Blueprint $table)
    {
        $table->renameColumn('mycol', 'old_mycol');
        $table->string('mycol', 100);
    });
}

当我运行它时，我收到错误：

[PDOException]
SQLSTATE[42S21]: 列已经存在：1060 重复的列名“mycol”

我最终将其拆分为 2 个单独的迁移，并且效果很好，但我不明白为什么一次性完成它是一个问题。

Answer 1

这是因为 Laravel 会在执行迁移时隐式地将任何添加新列或修改现有列的命令放在命令数组的开头。以下代码直接取自Illuminate\Database\Schema\Blueprint该类。

/**
 * Get the raw SQL statements for the blueprint.
 *
 * @param  \Illuminate\Database\Connection  $connection
 * @param  \Illuminate\Database\Schema\Grammars\Grammar  $grammar
 * @return array
 */
public function toSql(Connection $connection, Grammar $grammar)
{
    $this->addImpliedCommands();

    $statements = array();

    // Each type of command has a corresponding compiler function on the schema
    // grammar which is used to build the necessary SQL statements to build
    // the blueprint element, so we'll just call that compilers function.
    foreach ($this->commands as $command)
    {
        $method = 'compile'.ucfirst($command->name);

        if (method_exists($grammar, $method))
        {
            if ( ! is_null($sql = $grammar->$method($this, $command, $connection)))
            {
                $statements = array_merge($statements, (array) $sql);
            }
        }
    }

    return $statements;
}

/**
 * Add the commands that are implied by the blueprint.
 *
 * @return void
 */
protected function addImpliedCommands()
{
    if (count($this->getAddedColumns()) > 0 && ! $this->creating())
    {
        array_unshift($this->commands, $this->createCommand('add'));
    }

    if (count($this->getChangedColumns()) > 0 && ! $this->creating())
    {
        array_unshift($this->commands, $this->createCommand('change'));
    }

    $this->addFluentIndexes();
}

从上面的代码可以看出，在该toSql方法中，有一个调用addImpliedCommands，其中可能将几个命令添加到对象的命令数组的开头。这会导致mycol在重命名命令之前首先执行新列的命令。

要解决此问题，您实际上并不需要创建两个迁移。在同一次迁移中，您可以Schema::table()像这样简单地调用两次：

Schema::table('mytable', function(Blueprint $table)
{
    $table->renameColumn('mycol', 'old_mycol');
});

Schema::table('mytable', function(Blueprint $table)
{
    $table->string('mycol', 100);
});