如果将松散应用于猫鼬中使用聚合时不存在的字段会发生什么 [英] What will happen if unwind is applied to a field which is not present while using aggregation in mongoose

问题描述

假设我在字段上使用 $ unwind 在猫鼬模式中
根据以上链接，如果不存在该字段，则展开将忽略输入文档
我该如何防止这种情况，即使不存在该字段即不忽略输入文档，也无需取消处理即可返回查询结果

Lets say I am using a $unwind on a field in mongoose schema
According to the above link If field is not present then unwind will ignore input document
How can I prevent that so that result of query is returned without unwinding process even if field is not present i.e input document is not ignored

if a particular field exists   
  I want to do unwind and some more stuff (project and group) 
else
  I want input document no changes in this case  

推荐答案

根据上面的链接，如果不存在该字段，则展开将引发错误

According to the above link If field is not present then unwind will throw an error

不，不是:

• 如果由字段路径指定的字段中的值是不是数组，则db.collection.aggregate()会生成错误.
• 如果您为输入文档中不存在的字段指定了路径，则管道将忽略，并且不会为该输入文档输出文档.

• If a value in the field specified by the field path is not an array, db.collection.aggregate() generates an error.
• If you specify a path for a field that does not exist in an input document, the pipeline ignores the input document and will not output documents for that input document.

这是一个简单的例子:

> db.test.insert({a:[1]})
> db.test.insert({})

> db.test.aggregate({$unwind:'$a'})
{ "_id" : ObjectId("557362a17b97d77c38793c21"), "a" : 1 }
// no error -- but unwind only the document having the `a` field.

但是:

> db.test.insert({a:2})
> db.test.aggregate({$unwind:'$a'})
// will result in error 15978:
// """Value at end of $unwind field path '$a' must be an Array,
//    but is a NumberDouble"""

---

根据您的编辑，如果需要保留带有 missing 字段的文档以展开，则可以使用

---

As per your edit, if you need to keep documents with a missing field to unwind, you can $project a default value using $ifNull:

> db.test.find()
{ "_id" : ObjectId("557362a17b97d77c38793c21"), "a" : [ 1 ] }
{ "_id" : ObjectId("557362a57b97d77c38793c22") }

> db.test.aggregate([
    {$project: { a: { $ifNull: ['$a', [ null ]]}}},
    {$unwind: "$a"}
])
{ "_id" : ObjectId("557362a17b97d77c38793c21"), "a" : 1 }
{ "_id" : ObjectId("557362a57b97d77c38793c22"), "a" : null }

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