Query MongoDB by value when parent key is unknown

问题

I have a MongoDB collection called references that has a document where I need to return all the objects that have a key called format with the value of 1. An example of the document looks like this:

{
    "_id" : ObjectId("878f92ad6d9e8089aa3456a9"),
    "categories" : {
            "1" : {
                    "format" : 1,
                    ...
            }
    }
}

I have tried this:

db.references.find({
    "_id" :  ObjectId("878f92ad6d9e8089aa3456a9"), 
    "categories.$.format" : 1
}).pretty();

As well as this:

db.references.find({
    "_id" :  ObjectId("878f92ad6d9e8089aa3456a9"), 
    "categories.*.format" : 1
}).pretty();

and both of those are returning nothing.

回答1:

I think schema design is not quite well. Having a schema design like the following:

{
    "_id" : ObjectId("57fbe76f78c1638eaebfb21f"),
    "categories" : [
        {
            "cat_name" : 1,
            "format" : 1
        },
        {
            "cat_name" : 2,
            "format" : 6
        }
    ]
}

Makes much more sense, this way you can access the format field of the embedded category documents simply using dot notation. The query you requested can be: db.stackQuestion.find({ "categories.format": 1 })

回答2:

The mongoDB enables checking the existance of a field in the document by using the $exists element query operator.

The using of the property _id means that you want to extract specific document from the collection when you know its id so it can return at most 1 object. That is why you should not use the following line inside the query:

"_id" : ObjectId("878f92ad6d9e8089aa3456a9")

Simple query to get all the documents contains the categories field should look like this:

db.references.findAll( { 
  categories: { $exists: true } 
}).pretty();

By using the following query you get in one query all the results you wish to get:

db.references.find({ categories: { $exists:true }, 
                    "categories.$" : { $elemMatch: { "format": "1" } } 
}).pretty();

The $ is used for deep search for any element under categories.

For more information please read : $match , element match,

来源：https://stackoverflow.com/questions/39964798/query-mongodb-by-value-when-parent-key-is-unknown