如何在Laravel中将一个集合中的ID存储到另一个MongoDB集合中

Question

我有两个“收藏”类别和一个子类别。一个类别可以包含0个或多个子类别，这就是为什么我需要它们之间的关系，如何将_id或文档表单类别集合存储到子类别集合的文档中。

类别

{
    "_id" : ObjectId("5b8abe366a44fa2d080065d2"),
    "Title" : "Aptitude",
    "Description" : "Aptitude",
    "Slug" : "aptitude",
    "updated_at" : ISODate("2018-09-01T16:28:38Z"),
    "created_at" : ISODate("2018-09-01T16:28:38Z")
 }

子类别

{
    "_id" : ObjectId("5b9abbd06a44fa207c001674"),
    "Title" : "Aithmatic Apptitude",
    "Description" : "Aithmatic Apptitude",
    "Slug" : "arithmatic_aptitude",
    "CategoryId" : "5b8abe366a44fa2d080065d2",
    "updated_at" : ISODate("2018-09-13T19:34:40Z"),
    "created_at" : ISODate("2018-09-13T19:34:40Z")
}
{
    "_id" : ObjectId("5b9ac7e86a44fa207c001678"),
    "Title" : "Reasoning",
    "Description" : "Reasoning",
    "Slug" : "reasoning",
    "CategoryId" : "5b8abe366a44fa2d080065d2",
    "updated_at" : ISODate("2018-09-13T20:26:16Z"),
    "created_at" : ISODate("2018-09-13T20:26:16Z")
}

如您所见，我将category_id存储在子类别集合中的字符串"CategoryId" : "5b8abe366a44fa2d080065d2",

这是我的存储代码

$subcategory = Subcategory::create(
    [
        'Title' => $request->Title, 
        'Description' => $request->Description,
        'Slug' => $request->Slug,
        'CategoryId' =>  $request->CategoryId,
    ]

);

想要像实际参考文献"category_id" ObjectId("5b8abe366a44fa2d080065d2")那样痛。转换此命令db.subcategory.insert({Title:"Example",Description:"Example", Slug:"Example",CategoryId:db.category.find({_id:"5b9abc8cb3562ec4e990ec5b"})._id})怎么可能，请帮助