仅在ref不为null时尝试填充猫鼬-不起作用 [英] Trying to populate in mongoose only if ref is not null - not working

问题描述

我正在尝试获取包含其作者信息的书籍清单.

I'm trying to get a list of books with their author information.

某些用户已被删除，因此他们在数据库中不再有文档，因此，他们的信息为空.

Some users were deleted and therefore they have not longer a document in the db, therefore, their info is null.

如果他们的创作者仍然存在，我只会尝试拉书.

I'm trying to pull books ONLY if their creators still exist.

这是我的代码:

  Book.find({_creator:{$ne:null}}).populate(
        {
            path: '_creator',
            match: { _id: { $ne: null }}
        })
        .exec(function (err,books) {
        if(err) throw err;
        if(books) {
            res.send(books)
        }
    })

这是返回的内容:

  [
    {
        "_id":"55d98e6a4de71010099c59eb",
        "dateOfCreation":"2015-08-23T09:12:10.095Z",
        "_creator":null,
        "__v":0,
        "coverUrl":"cover14403211323926quv.png",
        "description":"asdasd",
        "name":"asdasd"
    }
]

请注意，_creator字段为null. 这是为什么?

Notice that the _creator field IS null. Why is that?

推荐答案

您需要了解代码的执行顺序:

you need to understand the order of execution of your code:

1. 猫鼬从数据库{_creator:{$ne:null}}中获取所有书籍. Mongo仅查看图书藏书中的参考资料来确定要退还哪些文件.您的书仍然引用了作者，而mongo不会注意到Authors集合中没有匹配的Author，因此已加载您的书.

1. mongoose gets all books from the database where {_creator:{$ne:null}}. Mongo is only looking at the reference inside the books collection to determine which documents to return. Your book still has a reference to an author, and mongo will not notice that there is no matching Author in the Authors collection, so your book is loaded.

猫鼬正在填充所有返回的结果:因此它将从Authors集合中加载作者，并将引用替换为真实对象.对于您的书，找不到匹配的作者，因此将null放在此处.

mongoose is populating all returned results: so it is loading the authors from the Authors collection and replaces the references with the real objects. For your book it does not find a matching author, so it puts the null there.

这就是为什么您最终得到结果列表的原因.

Thats why you end up with your resultlist.

Mongo不支持联接-因此，您无法进行包含多个集合中的数据的查询.填充只是将结果列表中的引用替换为真实数据的一种方法，您永远不能将填充的数据用作where子句的一部分.

Mongo does not support joins - therefore you cannot do a query that includes data from more than one collection. Populate is just a way to replace references in your resultlist with real data, you can never use populated data as part of your where clauses.

要解决您的问题，您可以:

To solve your issue you can either:

• 使用JS代码过滤最终结果列表，例如使用lodash库的_.filter.
• 更新所有图书，并在删除作者时删除参考书目.您可以在Author-Schema上使用钩子.

AuthorSchema.post('remove', function(doc) {// update your books here});

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