How to sort by 'value' of a specific key within a property stored as an array with k-v pairs in mongodb

问题

I have a mongodb collection, let's call it rows containing documents with the following general structure:

{
  "setid" : 154421,
  "date" : ISODate("2014-02-22T14:06:48.229Z"),
  "version" : 2,
  "data" : [
    {
      "k" : "name",
      "v" : "ryan"
    },
    {
      "k" : "points",
      "v" : "375"
    },
    {
      "k" : "email",
      "v" : "ryan@123.com"
    }
  ],
}

There is no guarantee what values of k and v might populate the "data" property for any particular document (eg. other documents might have 5 k-v pairs with different key names in it). The only rule is that documents with the same setid have the same k-v pairs. (i.e. the rows collection might hold 100 other documents with setid = 154421, that have the same set of 3 keys in the data property: "name", "points", "email", with their own respective values.

How would one, with this setup, construct a query to retrieve all rows with a particular setid sorted by points? I need, in effect, some way of saying 'sort by the the field data.v where the value of k==points or something like that...?

Something like this:

db.rows.find({setid:154421},{$sort:{'data.v',-1}, {$where: k:'points'}}})

I know this is the incorrect syntax, but I'm just taking a stab at it to illustrate my point.

Is it possible?

回答1:

Assuming that what you want would be all the documents that have the "points" value as a "key" in the array, and then sort on the "value" for that "key", then this is a little out of scope for the .find() method.

Reason being if you did something like this

db.collection.find({
    "setid": 154421, "data.k": "point" }
).sort({ "data.v" : -1 })

The problem is that even though the matched elements do have the matching key of "point", there is no way of telling which data.v you are referring to for the sort. Also, a sort within .find() results will not do something like this:

db.collection.find({
    "setid": 154421, "data.k": "point" }
).sort({ "data.$.v" : -1 })

Which would be trying to use a positional operator within a sort, essentially telling which element to use the value of v on. But this is not supported and not likely to be, and for the most likely explaination, that "index" value would be likely different in every document.

But this kind of selective sorting can be done with the use of .aggregate().

db.collection.aggregate([

    // Actually shouldn't need the setid
    { "$match": { "data": {"$elemMatch": { "k": "points" } } } },

    // Saving the original document before you filter
    { "$project": {
        "doc": {
           "_id": "$_id",
           "setid": "$setid",
           "date": "$date",
           "version": "$version",
           "data": "$data"
        },
        "data": "$data"
    }}

    // Unwind the array
    { "$unwind": "$data" },

    // Match the "points" entries, so filtering to only these
    { "$match": { "data.k": "points" } },

    // Sort on the value, presuming you want the highest
    { "$sort": { "data.v": -1 } },

    // Restore the document
    { "$project": {
        "setid": "$doc.setid",
        "date": "$doc.date",
        "version": "$doc.version",
        "data": "$doc.data"
    }} 

])

Of course that presumes the data array only has the one element that has the key points. If there were more than one, you would need to $group before the sort like this:

    // Group to remove the duplicates and get highest
    { "$group": { 
        "_id": "$doc",
        "value": { "$max": "$data.v" }
    }},

    // Sort on the value
    { "$sort": { "value": -1 } },

    // Restore the document
    { "$project": { 
        "_id": "$_id._id",
        "setid": "$_id.setid",
        "date": "$_id.date",
        "version": "$_id.version",
        "data": "$_id.data"
    }}

So there is one usage of .aggregate() in order to do some complex sorting on documents and still return the original document result in full.

Do some more reading on aggregation operators and the general framework. It's a useful tool to learn that takes you beyond .find().

来源：https://stackoverflow.com/questions/22289793/how-to-sort-by-value-of-a-specific-key-within-a-property-stored-as-an-array-wi