[mongodb] MongoDB distinct aggregation

I'm working on a query to find cities with most zips for each state:

db.zips.distinct("state", db.zips.aggregate([ {$group:{_id:{state:"$state", city:"$city"},numberOfzipcodes:{$sum:1}}}, {$sort:{numberOfzipcodes:-1}}]))

The aggregate part of the query seems to work fine, but when I add the distinct I get an empty result.

Is this because I have state in the id? Can I do something like distinct("_id.state ?

SQL Query: (group by & count of distinct)

select city,count(distinct(emailId)) from TransactionDetails group by city;

Equivalent mongo query would look like this:

db.TransactionDetails.aggregate([ 
{$group:{_id:{"CITY" : "$cityName"},uniqueCount: {$addToSet: "$emailId"}}},
{$project:{"CITY":1,uniqueCustomerCount:{$size:"$uniqueCount"}} } 
]);

You can use $addToSet with the aggregation framework to count distinct objects.

For example:

db.collectionName.aggregate([{
    $group: {_id: null, uniqueValues: {$addToSet: "$fieldName"}}
}])

You can call $setUnion on a single array, which also filters dupes:

{ $project: {Package: 1, deps: {'$setUnion': '$deps.Package'}}}