Mongodb: Aggregation : sum up values in an array before $group -

i have collection of documents following structure:

{     _id: 1,     array: [         {value: 10 },         {value: 11 },         {value: 12 }     ] } 

i want make aggregate query on collection: proportion of each item. (i.e. example proportion of item 1 value of item 1 divided sum of values of 3 items.

note: want within single query.

the basic idea here $unwind array, $group document , apply each array member. works better mongodb 2.6 or greater due $map operator:

db.collection.aggregate([     { "$unwind": "$array" },     { "$group": {        "_id": "$_id",        "array": { "$push": "$array" },        "total": { "$sum": "$array.value" }     }},     { "$project": {         "array": {             "$map": {                 "input": "$array",                 "as": "el",                 "in": {                     "value": "$$el.value",                     "prop": {                          "$divide": [ "$$el.value", "$total" ]                     }                 }             }         }     }} ]) 

or earlier versions:

db.collection.aggregate([     { "$unwind": "$array" },     { "$group": {        "_id": "$_id",        "array": { "$push": "$array" },        "total": { "$sum": "$array.value" }     }},     { "$unwind": "$array" },     { "$group": {         "_id": "$_id",         "array": {             "$push": {                 "value": "$array.value",                 "prop": {                     "$divide": [ "$array.value", "$total" ]                 }             }         }     }} ]) 

in either case, if not "aggregating" beyond document, far more efficient calculation in client code. $unwind here can costly due does.

also if stored "total" element, simple $project need, comes @ little cost itself. keeping total on updates simple usage of $inc operator $push new elements array.