Join/Intersection using mapReduce in MongoDB -

i want perform join operation on 2 distinct sets of documents having 1 field in common. have following setup.

collection a

{     "common_key" : "v1" ,     "a2" : "v2",     "a3" : "v3",     ..... } 

collection b

{     "common_key" : "v1" ,     "b2" : "z2",     "b3" : "z3",     ..... } 

since mapreduce can work on 1 collection merged contents of , b collection combo . additionally, added 1 more field _datatype indicate whether of type or b

collection combo = + b

{     "common_key" : "v1" ,     "b2" : "z2",     "b3" : "z3",     "_datatype" : "b",     ..... }  {     "common_key" : "v1" ,     "a2" : "v2",     "a3" : "v3",     "_datatype" : "a",     ..... } 

i want write mapreduce function such documents have same common_key each data type & b should output

{     "common_key" : v1 ,     "a" : [             {                "a2" : "v2",               "a3" : "v3",               ...              },              ...            ],     "b" : [             {                 "b2" : "z2",                 "b3" : "z3",                 .....            },            ..            ] } 

any pointers or appreciated

quick answer using aggregation framework

db.coll.aggregate({     "$group": {         "_id": "$common_key",         "a": {             "$push": {                 "$cond": [                     {                         "$eq": [                             "$_datatype",                             "a"                         ]                     },                     {                         "a2": "$a2",                         "a3": "$a3"                         // have put datatype field here                     },                      {}                 ]             }         },         "b": {             "$push": {                 "$cond": [                     {                         "$eq": [                             "$_datatype",                             "b"                         ]                     },                     {                         "b2": "$b2",                         "b3": "$b3"                         // have put b datatype field here                     },                     {}                 ]             }         }     } }) 

cons: have write fields of , b datatypes project them , you'll have , empty object on each array (a , b)