MongoDB - 检查文档中某个字段的值是否存在
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MongoDB - Check if value exists for a field in a document
提问by Pi Horse
I have a collection where the document looks like below:
我有一个集合，其中的文档如下所示：
/* 0 */
{
"_id" : ObjectId("5320b1c723bc746084fa7107"),
"channels" : [
3,
4
]
}
/* 1 */
{
"_id" : ObjectId("5320b1c723bc746084fa7107"),
"channels" : [ ]
}
I want to form a query such that I want all documents where channels has some value and is not empty.
我想形成一个查询，以便我想要所有通道具有某些值且不为空的文档。
I tried this:
我试过这个：
db.event_copy.find({"channels":{$exists:true}})
But that will still return me the documents with no values in channel.
但这仍然会返回通道中没有值的文档。
采纳答案by Pi Horse
I did it using this :
我是用这个做的：
db.event_copy.find({'channels.0' : {$exists: true}}).count()
回答by Neil Lunn
You need the $sizeoperator. To find something with noelements do the following
您需要$size运算符。要找到没有元素的东西，请执行以下操作
db.collection.find({ channels: {$size: 0} })
If you know you have a fixed size then do that
如果你知道你有一个固定的尺寸，那么就这样做
db.collection.find({ channels: {$size: 2} })
Otherwise reverse that with $not
否则用$not反转它
db.collection.find({ channels: {$not:{$size: 0}} })
And you can combine with $and:
你可以结合$and：
db.collection.find({ $and: [
{ channels: {$not:{$size: 0}} },
{ channels: {$exists: true } }
]})
