MongoDB Aggregation: Counting distinct fields - mongodb

Question

I am trying to write an aggregation to identify accounts that use multiple payment sources. Typical data would be.
{
account:"abc",
vendor:"amazon",
}
...
{
account:"abc",
vendor:"overstock",
}
Now, I'd like to produce a list of accounts similar to this
{
account:"abc",
vendorCount:2
}
How would I write this in Mongo's aggregation framework

Answer 1

I figured this out by using the $addToSet and $unwind operators.
Mongodb Aggregation count array/set size
db.collection.aggregate([
{
$group: { _id: { account: '$account' }, vendors: { $addToSet: '$vendor'} }
},
{
$unwind:"$vendors"
},
{
$group: { _id: "$_id", vendorCount: { $sum:1} }
}
]);
Hope it helps someone

Answer 2

I think its better if you execute query like following which will avoid unwind
db.t2.insert({_id:1,account:"abc",vendor:"amazon"});
db.t2.insert({_id:2,account:"abc",vendor:"overstock"});
db.t2.aggregate([
{ $group : { _id : { "account" : "$account", "vendor" : "$vendor" }, number : { $sum : 1 } } },
{ $group : { _id : "$_id.account", number : { $sum : 1 } } }
]);
Which will show you following result which is expected.
{ "_id" : "abc", "number" : 2 }

Answer 3

You can use sets
db.test.aggregate([
{$group: {
_id: "$account",
uniqueVendors: {$addToSet: "$vendor"}
}},
{$project: {
_id: 1,
vendorsCount: {$size: "$uniqueVendors"}
}}
]);

Answer 4

I do not see why somebody would have to use $group twice
db.t2.aggregate([ { $group: {"_id":"$account" , "number":{$sum:1}} } ])
This will work perfectly fine.

Answer 5

This approach doesn't make use of $unwind and other extra operations. Plus, this won't affect anything if new things are added into the aggregation. There's a flaw in the accepted answer. If you have other accumulated fields in the $group, it would cause issues in the $unwind stage of the accepted answer.
db.collection.aggregate([{
"$group": {
"_id": "$account",
"vendors": {"$addToSet": "$vendor"}
}
},
{
"$addFields": {
"vendorCount": {
"$size": "$vendors"
}
}
}])

Answer 6

To identify accounts that use multiple payment sources:
Use grouping to count data from multiple account records and group the result by account with count
Use a match case is to filter only such accounts having more than one payment method
db.payment_collection.aggregate([ { $group: {"_id":"$account" ,
"number":{$sum:1}} }, {
"$match": {
"number": { "$gt": 1 }
}
} ])
This will work perfectly fine,

Answer 7

db.UserModule.aggregate(
{ $group : { _id : { "companyauthemail" : "$companyauthemail", "email" : "$email" }, number : { $sum : 1 } } },
{ $group : { _id : "$_id.companyauthemail", number : { $sum : 1 } } }
);

Answer 8

An example
db.collection.distinct("example.item").forEach( function(docs) {
print(docs + "==>>" + db.collection.count({"example.item":docs}))
});