MongoDb "left join" with different column types - mongodb

Question

I have two collections: User and UserProfile.
UserProfile references User by the property userId as UUID.
User collection have _id field but as string.
How can I "left join" these two collections to find out what documents don't have a match?
Here is what I got so far:
db.UserProfile.aggregate({
$lookup:
{
from: 'User',
localField: 'userId', // this is UUID
foreignField: '_id', // this is string
as: 'user'
}
})
Here are two sample entities:
User:
{
"_id" : "0c7e18b2-3682-444d-a62b-30e311e76891",
"userName" : "programad",
"normalizedUserName" : "PROGRAMAD",
"email" : "programad#gmail.com",
"normalizedEmail" : "PROGRAMAD#GMAIL.COM",
"emailConfirmed" : true,
"concurrencyStamp" : "56d3d071-0e30-458a-880a-ed75e20863a9",
"lockoutEnabled" : true,
"roles" : [ ],
"claims" : [ ],
"logins" : [ ],
"tokens" : [ ],
"recoveryCodes" : [ ]
}
UserProfile:
{
"_id" : UUID("a4d5b6d5-f181-4267-5b81-08d64d02d681"),
"userId" : UUID("0c7e18b2-3682-444d-a62b-30e311e76891"),
"createDate" : ISODate("2018-08-01T00:00:00.000-03:00"),
"type" : "Personal",
"name" : "Daniel Gomes",
"motto" : "Code Lover",
"bio" : "bio yadda yadda",
"studioName" : "My studio name",
"location" : "Santos/Brazil",
"externalLinks" : [ ]
}

Answer 1

Ok, a gentleman called Vignan Yarlagadda helped me on this.
I needed to find wich profiles lack users or vice-versa.
He came around with this:
db.UserMissing.drop();
var cur = db.UserProfile.find({"_id":{"$exists":1}}).addOption(16);
cur.forEach(function(x){
db.UserMissing.insert({"_id":x.userId,"userId":x.userId})
});
var cur = db.User.find({"_id":{"$exists":1}}).addOption(16);
cur.forEach(function(x){
var string=x._id;
var y=UUID(string);
db.UserProfile.find({"userId":y}).forEach(function(doc){
db.UserMissing.update({"userId":doc.userId},{"$set":{"Matching":true}},{"multi":true});
});
});
db.UserMissing.find({"Matching":{"$exists":0}});
It creates **a temporary collection** called UserMissing and I can see witch ones to focus my attention.