Mongodb - aggregation $push if conditional

Question

I am trying to aggregate a batch of documents. There are two fields in the documents I would like to $push. However, lets say they are "_id" and "A" fields, I only want $push "_id" and "A" if "A" is $gt 0.

I tried two approaches.

First one.

db.collection.aggregate([{ "$group":{     "field": {         "$push": {             "$cond":[                 {"$gt":["$A", 0]},                 {"id": "$_id", "A":"$A"},                 null             ]         }     },     "secondField":{"$push":"$B"} }]) 

But this will push a null value to "field" and I don't want it.

Second one.

db.collection.aggregate([{ "$group":     "field": {         "$cond":[             {"$gt",["$A", 0]},             {"$push": {"id":"$_id", "A":"$A"}},             null         ]     },     "secondField":{"$push":"$B"} }]) 

The second one simply doesn't work...

Is there a way to skip the $push in else case?

ADDED:

Expected documents:

{     "_id":objectid(1),     "A":2,     "B":"One" }, {     "_id":objectid(2),     "A":3,     "B":"Two" }, {     "_id":objectid(3),     "B":"Three" } 

Expected Output:

{     "field":[         {             "A":"2",             "_id":objectid(1)         },         {             "A":"3",             "_id":objectid(2)         },     ],     "secondField":["One", "Two", "Three"] } 

Answer 1

You can use "$$REMOVE":

• This system variable was added in version 3.6 (mongodb docs)

db.collection.aggregate([{    $group:{        field: {           $push: {               $cond:[                 { $gt: ["$A", 0] },                 { id: "$_id", A:"$A" },                 "$$REMOVE"             ]           }         },         secondField:{ $push: "$B" }     } ]) 

In this way you don't have to filter nulls.

Answer 2

This is my answer to the question after reading the post suggested by @Veeram

db.collection.aggregate([{ "$group":{     "field": {         "$push": {             "$cond":[                 {"$gt":["$A", 0]},                 {"id": "$_id", "A":"$A"},                 null             ]         }     },     "secondField":{"$push":"$B"} }, {     "$project": {         "A":{"$setDifference":["$A", [null]]},         "B":"$B"     } }])