node.js - Returning a subset of array items in MongoDB -

can offer advice on how return subset of array items? example, let's suppose have collection of documents (similar example below) contains simple _id key , key contains array of objects.

i find _id's , matching objects match simple criteria:

// campaigns {   "_id" : "fred's c25k",   "campaigndata" : [     {       "date" : "2015-06-17",       "source" : "nike"     },     {       "date" : "2015-06-17",       "source" : "reebok",     },     {       "date" : "2015-06-12",       "source" : "nike"     },     {       "date" : "2015-06-14",       "source" : "adidas"     },   ] }, {   "_id" : "mike's marathon",   "campaigndata" : [     {       "date" : "2015-06-17",       "source" : "nike"     }   ] }, {   "_id" : "jacob's jamboree",   "campaigndata" : [     {       "date" : "2015-06-17",       "source" : "keen"     }   ] } 

i result contain _id , matching objects for, say, date value of "2015-06-17"

// goal => generate result set looks like: {   "_id" : "fred's c25k",   "campaigndata" : [     {       "date" : "2015-06-17",       "source" : "nike"     },     {       "date" : "2015-06-17",       "source" : "reebok",     }   ] }, {   "_id" : "mike's marathon",   "campaigndata" : [     {       "date" : "2015-06-17",       "source" : "nike"     }   ] }, {   "_id" : "jacob's jamboree",   "campaigndata" : [     {       "date" : "2015-06-17",       "source" : "keen"     }   ] } 

use aggregation framework achieve desired result. following pipeline consists of $match operator stage first step filter documents should pass through pipeline. next stage $unwind operator deconstructs campaigndata array field input documents output document each element. each output document replaces array element value. need $match step subdocuments match given date criteria. matched documents used in next $group pipeline step preserve original array using $push group accumulator operator:

db.collection.aggregate([     {         "$match": {             "campaigndata.date" : "2015-06-17"         }     },     {         "$unwind": "$campaigndata"     },     {         "$match": {             "campaigndata.date" : "2015-06-17"         }     },     {         "$group": {             "_id": "$_id",             "campaigndata": {                 "$push": "$campaigndata"             }         }     } ]) 

result:

/* 0 */ {     "result" : [          {             "_id" : "mike's marathon",             "campaigndata" : [                  {                     "date" : "2015-06-17",                     "source" : "nike"                 }             ]         },          {             "_id" : "jacob's jamboree",             "campaigndata" : [                  {                     "date" : "2015-06-17",                     "source" : "keen"                 }             ]         },          {             "_id" : "fred's c25k",             "campaigndata" : [                  {                     "date" : "2015-06-17",                     "source" : "nike"                 },                  {                     "date" : "2015-06-17",                     "source" : "reebok"                 }             ]         }     ],     "ok" : 1 }