Find duplicates across multiple MongoDb collections - mongodb

Question

Coming from MySQL, I am wondering, how can we find duplicates across multiple collections in MongoDB ?
Let say I have two (or more) collections :
human :
_id
firstname
cat:
_id
nickname
What would be an efficient solution to list duplicated names. This includes if a name is used by 2+ users only, by 2+ cats only, or by at least one user and one cat. Our result should therefor contains duplicates of both collections AND duplicates across those collections (cats and humans with the same name)
Expected result :
The list of the duplicated values, the number of occurence could be interesting but is not essential.
Question is not about whether or not the proposed db schema would be appropriate in this situation, but about the best MongoDB solution.
Edit
My description of a duplicate was not really what I intended it, if it is not existing in one collection but is duplicated in another collection it still is a duplicate

Answer 1

For two collections with MongoDB 3.2 you can use $lookup aggregation (it's equivalent to left outer join you have used in MySQL):
db.human.aggregate([
{$group: {_id: "$firstname"}},
{$lookup: {
from: "cat",
localField: "_id",
foreignField: "nickname",
as: "cats"
}
},
{$match:{cats:{$ne:[]}}},
{$project: {catsCount:{$size:"$cats"}}}])
Stages:
Group humans by name as there could be humans with same name
Attach to each group array of cats which have same nickname as group id (i.e. human firstname)
Filter out those humans which don't have any matches in cats collection
Project result to get only names and count of matches
Result will look like
[
{ _id: "Bob", catsCount: 2 },
{ _id: "Alex", catsCount: 1 }
]
NOTE: If you need to join several collections, you can apply $lookup stage several times.

Answer 2

A solution I found :
Combine data in one collection
Find duplicates in the collection
And export it in a table
Code :
mapHuman = function() {
var values = {
name: this.firstname
};
emit(this._id, values);
};
mapCat = function() {
var values = {
name: this.name
};
emit(this._id, values);
};
reduce = function(k, values) {
var result = {names: []};
values.forEach(function(value) {
result.names.push(value.slug);
});
return result;
};
db.human.mapReduce(mapUsers, reduce, {"out": {"reduce": "name"}});
db.cat.mapReduce(mapUsers, reduce, {"out": {"reduce": "name"}});
db.name.aggregate(
{"$group" : { "_id": "$value.name", "count": { "$sum": 1 } } },
{"$match": {"_id" :{ "$ne" : null } , "count" : {"$gt": 1} } },
{"$project": {"name" : "$_id", "_id" : 0}},
{"$out": "duplicate"}
)
db.duplicate.find()