Laravel Schema onDelete设置为null

无法弄清楚如何在Laravel中的表上设置正确的onDelete约束.(我和SqLite合作)

$table->...->onDelete('cascade'); // works
$table->...->onDelete('null || set null'); // neither of them work

我有3次迁移,创建了gallery表:

Schema::create('galleries', function($table)
{
    $table->increments('id');
    $table->string('name')->unique();
    $table->text('path')->unique();
    $table->text('description')->nullable();
    $table->timestamps();
    $table->engine = 'InnoDB';
});

创建图片表:

Schema::create('pictures', function($table)
{
    $table->increments('id');
    $table->text('path');
    $table->string('title')->nullable();
    $table->text('description')->nullable();
    $table->integer('gallery_id')->unsigned();
    $table->foreign('gallery_id')
        ->references('id')->on('galleries')
        ->onDelete('cascade');
    $table->timestamps();
    $table->engine = 'InnoDB';
});

将图库表链接到图片:

Schema::table('galleries', function($table)
{
    // id of a picture that is used as cover for a gallery
    $table->integer('picture_id')->after('description')
        ->unsigned()->nullable();
    $table->foreign('picture_id')
        ->references('id')->on('pictures')
        ->onDelete('cascade || set null || null'); // neither of them works
});

我没有收到任何错误.此外,即使"级联"选项也不起作用(仅在图库表上).删除图库会删除所有图片.但删除封面图片,不会删除图库(用于测试目的).

由于即使"级联"未被触发,我"设置空"也不是问题.

编辑(解决方法):

阅读完这篇文章之后,我改变了我的架构.现在,图片表包含一个"is_cover"单元格,用于指示此图片是否是其相册的封面.

原始问题的解决方案仍然受到高度赞赏!