如何在 mongodb 中查询子对象

我是新手，正在尝试查询子对象。我有一个国家的集合，每个国家有儿童城市。其中一个城市的 Name 属性为 null，这会导致我的应用程序出错。我如何查询国家集合来查找名称为 = = null 的子城市？

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最佳答案

If it is exactly null (as opposed to not set):

db.states.find({"cities.name": null})

(but as javierfp points out, it also matches documents that have no cities array at all, I'm assuming that they do).

If it's the case that the property is not set:

db.states.find({"cities.name": {"$exists": false}})

I've tested the above with a collection created with these two inserts:

db.states.insert({"cities": [{name: "New York"}, {name: null}]})
db.states.insert({"cities": [{name: "Austin"}, {color: "blue"}]})

The first query finds the first state, the second query finds the second. If you want to find them both with one query you can make an $or query:

db.states.find({"$or": [
{"cities.name": null},
{"cities.name": {"$exists": false}}
]})

Assuming your "states" collection is like:

{"name" : "Spain", "cities" : [ { "name" : "Madrid" }, { "name" : null } ] }
{"name" : "France" }

The query to find states with null cities would be:

db.states.find({"cities.name" : {"$eq" : null, "$exists" : true}});

It is a common mistake to query for nulls as:

db.states.find({"cities.name" : null});

because this query will return all documents lacking the key (in our example it will return Spain and France). So, unless you are sure the key is always present you must check that the key exists as in the first query.