how to solve this error in mongodb aggregation: TypeError: ({$unwind:{path:"$tags", preserveNullAndEmptyArrays:true}}) is not iterable : - mongodb-query

Question

i have a collection
[{
"_id": {"$oid": "63873b95c7e956270e734f3c"},
"employeeId": "emp1",
"users": [{"name": "Allen","registered": true},
{"name": "Henry","registered": true},
{"name": "Adam","registered": false}],
"tags": ["good","excellent","average"]
},{
"_id": {"$oid": "63873c05c7e956270e734f3d"},
"employeeId": "emp2",
"users": [{"name": "Federick","registered": false},
{"name": "Mary","registered": true},
{"name": "Sam","registered": false} ],
"tags": ["poor","excellent", "good"]}
,{
"_id": {"$oid": "63873c1fc7e956270e734f3e"},
"employeeId": "emp3",
"users": [ {"name": "john", "registered": true},
{"name": "jack","registered": true},
{"name": "elle", "registered": false } ],
"tags": ["very good", "excellent", "good" ]}]
i am trying to groupby tags and employee with count of employees under each tag,expected output is
i am getting the correct output by python code which is
pipeline = [{"$unwind":"$users"},{"$match":{"users.registered":True}},
{"$unwind":"$tags"},
{
"$group": {
"_id": "$tags","employees":{"$push": {"employeeId":"$employeeId"}},
"employees" : { "$addToSet" : "$employeeId" },
"count": {
"$sum": 1
}
}
},
{"$project":{"_id":1,"employees":1,"size":{"$size":"$employees"}}},
{"$sort" : { "size": -1 } },
]
rec = db.ratings.aggregate(pipeline)
but its giving error in mongoshell

Answer 1

Is this what you want?
db.collection.aggregate([
{
"$unwind": "$tags"
},
{
"$group": {
"_id": "$tags",
"employees": {
"$push": "$employeeId"
}
}
},
{
"$project": {
"_id": 1,
"employees": 1,
"size": {
"$size": "$employees"
}
}
},
{
"$sort": {
"size": -1
}
}
])
See how it works on the playground example