MongoDB: How to match on the elements of an array? - mongodb

Question

I have two collections as follows:
db.qnames.find()
{ "_id" : ObjectId("5a4da53f97a9ca769a15d49e"), "domain" : "mail.google.com", "tldOne" : "google.com", "clients" : 10, "date" : "2016-12-30" }
{ "_id" : ObjectId("5a4da55497a9ca769a15d49f"), "domain" : "mail.google.com", "tldOne" : "google.com", "clients" : 9, "date" : "2017-01-30” }
and
db.dropped.find()
{ "_id" : ObjectId("5a4da4ac97a9ca769a15d49c"), "domain" : "google.com", "dropDate" : "2017-01-01", "regStatus" : 1 }
I would like to join the two collections and choose the documents for which 'dropDate' field (from dropped collection) is larger than the 'date' filed (from qnames field). So I used the following query:
db.dropped.aggregate( [{$lookup:{ from:"qnames", localField:"domain",foreignField:"tldOne",as:"droppedTraffic"}},
{$match: {"droppedTraffic":{$ne:[]} }},
{$unwind: "$droppedTraffic" } ,
{$match: {dropDate:{$gt:"$droppedTraffic.date"}}} ])
but this query does not filter the records where dropDate < date. Anyone can give me a clue of why it happens?

Answer 1

The reason why you are not getting the record is
Date is used as a String in your collections, to make use of the comparison operators to get the desired result modify your collection documents using new ISODate("your existing date in the collection")
Please note even after modifying both the collections you need to modify your aggregate query, since in the final $match query two values from the same document is been compared.
Sample query to get the desired documents
db.dropped.aggregate([
{$lookup: {
from:"qnames",
localField:"domain",
foreignField:"tldOne",
as:"droppedTraffic"}
},
{$project: {
_id:1, domain:1,
regStatus:1,
droppedTraffic: {
$filter: {
input: "$droppedTraffic",
as:"droppedTraffic",
cond:{ $gt: ["$$droppedTraffic.date", "$dropDate"]}
}
}
}}
])
In this approach given above we have used $filter which avoids the $unwind operation

Answer 2

You should use $redact to compare two fields of the same document. Following example should work:
db.dropped.aggregate( [
{$lookup:{ from:"qnames", localField:"domain",foreignField:"tldOne",as:"droppedTraffic"}},
{$match: {"droppedTraffic":{$ne:[]} }},
{$unwind: "$droppedTraffic" },
{
"$redact": {
"$cond": [
{ "$lte": [ "$dropDate", "$droppedTraffic.date" ] },
"$$KEEP",
"$$PRUNE"
]
}
}
])