Php Laravel Schema onDelete 设置为空

无法理解如何在Laravel中对表设置正确的onDelete约束.(我正在与SqLite合作)

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

我有3个迁移，正在创建图片库表格:

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';
});

正在创建图片表格:

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，即使是"层叠"选项也不起作用(仅在 gallery 桌子上).删除图片库将删除所有图片.但是删除封面图片并不会删除图库(出于测试目的).

因为连"级联"都没有触发，所以我"设为空"不是问题.

EDIT (workaround):

在读完这article篇文章后，我稍微改变了一下我的模式.现在，Pictures表包含一个"is_cover"单元格，表示此图片是否为其相册的封面.

我们仍然高度赞赏解决原来问题的方法！