MongoDB: Find document given field values in an object with an unknown key

问题

I'm making a database on theses/arguments. They are related to other arguments, which I've placed in an object with a dynamic key, which is completely random.

{
    _id : "aeokejXMwGKvWzF5L",
    text : "test",
    relations : {
        cF6iKAkDJg5eQGsgb : {
            type : "interpretation",
            originId : "uFEjssN2RgcrgiTjh",
            ratings: [...]
         }
    }
}

Can I find this document if I only know what the value of type is? That is I want to do something like this:

db.theses.find({relations['anything']: { type: "interpretation"}}})

This could've been done easily with the positional operator, if relations had been an array. But then I cannot make changes to the objects in ratings, as mongo doesn't support those updates. I'm asking here to see if I can keep from having to change the database structure.

回答1:

Though you seem to have approached this structure due to a problem with updates in using nested arrays, you really have only caused another problem by doing something else which is not really supported, and that is that there is no "wildcard" concept for searching unspecified keys using the standard query operators that are optimal.

The only way you can really search for such data is by using JavaScript code on the server to traverse the keys using $where. This is clearly not a really good idea as it requires brute force evaluation rather than using useful things like an index, but it can be approached as follows:

db.theses.find(function() {
    var relations = this.relations;
    return Object.keys(relations).some(function(rel) {
        return relations[rel].type == "interpretation";
    });
))

While this will return those objects from the collection that contain the required nested value, it must inspect each object in the collection in order to do the evaluation. This is why such evaluation should really only be used when paired with something that can directly use an index instead as a hard value from the object in the collection.

Still the better solution is to consider remodelling the data to take advantage of indexes in search. Where it is neccessary to update the "ratings" information, then basically "flatten" the structure to consider each "rating" element as the only array data instead:

{
    "_id": "aeokejXMwGKvWzF5L",
    "text": "test",
    "relationsRatings": [
        {
            "relationId": "cF6iKAkDJg5eQGsgb",
            "type": "interpretation",
            "originId": "uFEjssN2RgcrgiTjh",
            "ratingId": 1,
            "ratingScore": 5
        },
        {
            "relationId": "cF6iKAkDJg5eQGsgb",
            "type": "interpretation",
            "originId": "uFEjssN2RgcrgiTjh",
            "ratingId": 2,
            "ratingScore": 6
        }
   ]
}

Now searching is of course quite simple:

db.theses.find({ "relationsRatings.type": "interpretation" })

And of course the positional $ operator can now be used with the flatter structure:

db.theses.update(
    { "relationsRatings.ratingId": 1 },
    { "$set": { "relationsRatings.$.ratingScore": 7 } }
)

Of course this means duplication of the "related" data for each "ratings" value, but this is generally the cost of being to update by matched position as this is all that is supported with a single level of array nesting only.

So you can force the logic to match with the way you have it structured, but it is not a great idea to do so and will lead to performance problems. If however your main need here is to update the "ratings" information rather than just append to the inner list, then a flatter structure will be of greater benefit and of course be a lot faster to search.

来源：https://stackoverflow.com/questions/34520630/mongodb-find-document-given-field-values-in-an-object-with-an-unknown-key