Combine three different Aggregate into a single one - mongodb

Question

I have three different Aggregations that I would like to combine to one.
objsCount = self.db.bidding.aggregate([
{ "$group": {
"_id": "$bid_item",
"number_of_objects": { "$sum": 1 }
}}
])
objsPrice = self.db.bidding.aggregate([
{"$match": {"$and" : [{"price": {"$ne":"null"}},{"users":{"$ne":"null"}}]}},
{ "$group": {
"_id": "$bid_item",
"total_price": { "$sum": '$price' },
"total_users": { "$sum": '$users' }
}}
])
objsHigestBid = self.db.bidding.aggregate([
{"$match": {"$and" : [{"highestBid": {"$ne":"null"}},{"users":{"$ne":"null"}}]}},
{ "$group": {
"_id": "$bid_item",
"total_number_of_users_after_bid": { "$sum": '$highestBid' }
}}
])
The output I'm looking for is:
[{ _id : '',
number_of_objects : '',
total_price : '',
total_users : '',
total_number_of_users_after_bid : ''
},...]
How could I change my "aggregates" to one single "aggregate" that make this possible?

Answer 1

With a single $group pipeline, you can take advantage of the $cond operator in the pipeline step to evaluate the counts based on the logic specified, something like the following:
db.bidding.aggregate([
{
"$group": {
"_id": "$bid_item",
"number_of_objects": { "$sum": 1 },
"total_price": {
"$sum": {
"$cond": [ { "$ne": [ "$price", "null" ] }, "$price", 0 ]
}
},
"total_users": {
"$sum": {
"$cond": [ { "$ne": [ "$users", "null" ] }, "$users", 0 ]
}
},
"total_number_of_users_after_bid": {
"$sum": {
"$cond": [
{
"$and": [
{ "$ne": [ "$highestBid", "null" ] },
{ "$ne": [ "$users", "null" ] }
]
},
"$highestBid", 0
]
}
}
}
}
])