How do I sort a collection based on values in an array

问题

I have collection named result with following values:-

> db.result.findOne()
{
        "_id" : ObjectId("53b05264421aa97e980ba404"),
        "result" : [
                {
                        "attempted" : 49,
                        "subject_total_marks" : 50,
                        "score" : 15,
                        "correct_subject_answer" : 15,
                        "subject" : "Biology"
                },
                {
                        "attempted" : 30,
                        "subject_total_marks" : 30,
                        "score" : 4,
                        "correct_subject_answer" : 4,
                        "subject" : "Chemistry"
                },
                {
                        "attempted" : 20,
                        "subject_total_marks" : 20,
                        "score" : 7,
                        "correct_subject_answer" : 7,
                        "subject" : "Physics"
                },
                {
                        "attempted" : 99,
                        "correct_subject_answer" : 26,
                        "score" : 26,
                        "subject_total_marks" : 100,
                        "subject" : "Total"
                }
        ],
        "useruid" : NumberLong(548),
        "exam_code" : 301,
        "ess_time" : 1404062120
}

Now I want to sort the collection based on "score" of "total" which is inside result array. Then sort based on score of physics, mathematics and then based on chemistry and finally biology.

回答1:

As you may have already tried, you cannot specify a specific item inside an array as a "key" to "sort" with a simple find. For this you are going to need the aggregate method in order to get the keys you want to sort on.

db.exam.aggregate([

     # Unwind to de-normalize
     { "$unwind": "$result" },

     # Group back to the document and extract each score
     { "$group": {
         "_id": "$_id",
         "result": { "$push": "$result" },
         "useruid": { "$first": "$useruid" },
         "exam_code": { "$first": "$exam_code" },
         "ess_time": { "$first": "$ess_time" },
         "Total": { 
             "$max": {
                 "$cond": [
                     { "$eq": [ "$result.subject", "Total" ] },
                     "$result.score",
                     0
                 ]
             }
         },
         "Physics": { 
             "$max": {
                 "$cond": [
                     { "$eq": [ "$result.subject", "Physics" ] },
                     "$result.score",
                     0
                 ]
             }
         },
         "Mathematics": { 
             "$max": {
                 "$cond": [
                     { "$eq": [ "$result.subject", "Mathematics" ] },
                     "$result.score",
                     0
                 ]
             }
         },
         "Chemistry": { 
             "$max": {
                 "$cond": [
                     { "$eq": [ "$result.subject", "Chemistry" ] },
                     "$result.score",
                     0
                 ]
             }
         },
         "Biology": { 
             "$max": {
                 "$cond": [
                     { "$eq": [ "$result.subject", "Biology" ] },
                     "$result.score",
                     0
                 ]
             }
         }
     }},

     # Sort on those scores
     { "$sort": {
         "Total": -1,
         "Physics": -1,
         "Mathematics": -1,
         "Chemistry": -1,
         "Biology": -1
     }},

     # Project final wanted fields
     { "$project": {
         "result": 1,
         "useruid": 1,
         "exam_code": 1,
         "ess_time": 1
     }}
])

So here you "extract" the matching values using the $cond operator within a $max statement after unwinding the array. The de-normalized documents do not all have the same values as they now represent the items in the array, so you test them.

With those extracted keys you can sort your whole documents again, and then finally discard those fields as you no longer need them.

来源：https://stackoverflow.com/questions/24644005/how-do-i-sort-a-collection-based-on-values-in-an-array