Mongo aggregation pipeline : How can I access one group stage variable in subsequent stage? - mongodb

Question

I have two collections here :
> db.Unit.find({}).limit(1).pretty()
{
"_id" : ObjectId("58b6878de1648d05903e1beb"),
"number" : "07in15in4",
"owner" : {
"name" : "vacant"
},
"floor" : 15,
"tower" : 4,
"VisitorLogs" : [
ObjectId("58b6878ee1648d05903e1d22"),
ObjectId("58b6878ee1648d05903e236e"),
ObjectId("58b6878ee1648d05903e23c9"),
ObjectId("58b6878ee1648d05903e2454"),
ObjectId("58b6878ee1648d05903e2915"),
ObjectId("58b6878ee1648d05903e2aae"),
ObjectId("58b6878ee1648d05903e2b93")
]
}
and
> db.VisitorLog.find({}).limit(1).pretty()
{ "_id" : ObjectId("58b6878fe1648d05903e2efa"), "purpose" : "WorkHere" }
VisitorLogs in the Unit collection refers to the second collection, i.e. VisitorLog.
I need to find the the units which have the maximum number of visits for each purpose.
I tried this in mongo shell:
units=db.Unit
units.aggregate([
{$match:{'VisitorLogs':{$gt:[]}}},
{$unwind:'$VisitorLogs'},
{$lookup:{
from:"VisitorLog",
localField:"VisitorLogs",
foreignField:"_id",
as:"log"}
},
{$group:{_id:{number:"$number", purpose:"$log.purpose"}, count:{$sum:1}}},
{$sort:{count:-1}},
{$group:{_id:"$_id.purpose", unit:{$first:"$_id.number"}}},
{$limit:10}
])
I get the following result :
{ "_id" : [ "Business" ], "unit" : "05in12in2" }
{ "_id" : [ "WorkHere" ], "unit" : "09in04in2" }
{ "_id" : [ "Casual" ], "unit" : "10in05in2" }
{ "_id" : [ "JobInterview" ], "unit" : "05in14in2" }
This means that, for example, unit number 05in12in2 had the maximum visits which had the purpose "Business"
I now want to get the number of "business" visits that 05in12in2 had.
I think its in "count" variable of the first group stage.
How do I access that ? I tried {$group:{_id:"$_id.purpose", unit:{$first:"$_id.number"}, visits:"$count"}}, in the second last stage, i.e just before the limit stage but I get error :
> units.aggregate([
... {$match:{'VisitorLogs':{$gt:[]}}},
... {$unwind:'$VisitorLogs'},
... {$lookup:{from:"VisitorLog", localField:"VisitorLogs", foreignField:"_id", as:"log"}},
... {$group:{_id:{number:"$number", purpose:"$log.purpose"}, count:{$sum:1}}},
... {$sort:{count:-1}},
... {$group:{_id:"$_id.purpose", unit:{$first:"$_id.number"}, visits:"$count"}},
... {$limit:10}
... ])
assert: command failed: {
"ok" : 0,
"errmsg" : "the group aggregate field 'visits' must be defined as an expression inside an object",
"code" : 15951
} : aggregate failed
_getErrorWithCode#src/mongo/shell/utils.js:25:13
doassert#src/mongo/shell/assert.js:13:14
assert.commandWorked#src/mongo/shell/assert.js:287:5
DBCollection.prototype.aggregate#src/mongo/shell/collection.js:1312:5
#(shell):1:1
Can someone please help me ?

Answer 1

You have this information, but you are cutting it off by the $group stage. Just change the stage to the following (notice the last argument):
{ $group : {
_id : "$_id.purpose",
unit : { $first : "$_id.number" },
visits : { $first : "$count" }
}}

Answer 2

The following outputs what I needed :
units.aggregate([
{$match:{'VisitorLogs':{$gt:[]}}},
{$unwind:'$VisitorLogs'},
{$lookup:{from:"VisitorLog", localField:"VisitorLogs", foreignField:"_id", as:"log"}},
{$group:{_id:{number:"$number", purpose:"$log.purpose"}, count:{$sum:1}}},
{$sort:{count:-1}},
{$group:{_id:"$_id.purpose", number:{$first:"$_id.number"}, visits:{$first:"$count"}}},
{$limit:10}
])