如何在mongodb中查询子对象 [英] how to query child objects in mongodb

问题描述

我是mongodb的新手，正在尝试查询子对象.我有一个州的集合，每个州都有一个儿童城市.其中一个城市的Name属性为null，这会导致我的应用程序出现错误.我将如何查询州集合以查找名称为== null的子城市?

I'm new to mongodb and am trying to query child objects. I have a collection of States, and each State has child Cities. One of the Cities has a Name property that is null, which is causing errors in my app. How would I query the State collections to find child Cities that have a name == null?

推荐答案

如果恰好是null(而不是未设置):

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

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

(但正如javierfp所指出的，它也匹配完全没有城市数组的文档，我假设它们确实匹配).

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

如果未设置该属性，则:

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

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

我已经用由这两个插入片段创建的集合测试了以上内容:

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"}]})

第一个查询找到第一个状态，第二个查询找到第二个状态.如果要通过一个查询找到它们两者，可以进行$or查询:

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}}
]})

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