aggregation framework - mongodb - filter out some values when doing $unwind -

i have following customer order data in mongodb

 "_id" : 7,  "customer name" : "john smith",  "orderitem" : [          {                  "product_category" : "mobile",                  "price" : 900          },          {                  "product_category" : "computer",                  "price" : 4200.48          },          {                  "product_category" : "tv",                  "price" : 670.20          },          {                  "product_category" : "tv",                  "price" : 960.52          }  ] 

i need average each product category this:

 "_id" : 7,  "customer name" : "john smith",  "orderitem" : [          {                  "product_category" : "mobile",                  "price" : 900          },          {                  "product_category" : "computer",                  "price" : 4200.48          },          {                  "product_category" : "tv",                  "price" : 815.36          }  ] 

i tried use $unwind not sure how group them . ?

use aggregation framework pipeline consists of following stages: $match operation in first pipeline stage filters document stream allow matching documents (document _id = 7 in case) pass unmodified next pipeline stage, $unwind operation. deconstructs desired orderitem array field input documents output document each element can group on , aggregation operation of finding average of category prices. next stage in pipeline $group operation groups input documents product_category , applies $avg expression each group on price. last stage $project reshapes each document in stream produce desired outcome. aggregation like:

db.collection.aggregate([     {         "$match": {"_id": 7}     },     {            "$unwind": "$orderitem"     },     {         "$group": {             "_id": "$orderitem.product_category",             "average_price": {                 "$avg": "$orderitem.price"             }         }     },     {         "$project": {             "_id": 0,             "product_category" : "$_id",             "average_price": 1         }      } ]) 

result:

{     "result" : [          {             "average_price" : 4200.48,             "product_category" : "computer"         },          {             "average_price" : 815.36,             "product_category" : "tv"         },          {             "average_price" : 900,             "product_category" : "mobile"         }     ],     "ok" : 1 }