MongoDB Aggregation Pipeline: $match with expression not possible? - mongodb

Question

I'm doing a rather complicated aggregation pipeline and have a rather strange phenomenon - I extracted a short example to visualize my problem here.
It seemed related to MongoDb $addFields and $match - but it doesn't contain any information for me to fix the problem at hand.
Note: Please note that my problem is not with the specific example of using date fields and or dealing with values, the problem is that I'm not able to $match using an expression - using a field that was added before with $addFields or not.
Given MongoDB: 3.6.3 (currently latest)
Let's insert some testdata:
db.testexample.insert({
"dateField": new ISODate("2016-05-18T16:00:00Z")
});
db.testexample.insert({
"dateField": new ISODate("2018-05-18T16:00:00Z")
});
Now let's make simple pipeline that computes only the year of the date and $matches on that:
db.testexample.aggregate([
{
"$addFields": {
"dateFieldYear": {"$year": "$dateField"}
}
},
{
"$match": {
"dateFieldYear": {"$eq": "$dateFieldYear"}}
}
}
])
--> No matches
It should match as it's the same field? Maybe with more trickery (using an $add)?
db.testexample.aggregate([
{
"$addFields": {
"dateFieldYear": {"$year": "$dateField"}
}
},
{
"$match": {
"dateFieldYear": {"$eq": {"$add": ["$dateFieldYear", 0]}}
}
}
])
--> No matches
Still no dice.. Next i thought that variables altogether are a problem. So let's fix the values:
db.testexample.aggregate([
{
"$addFields": {
"dateFieldYear": {"$year": "$dateField"}
}
},
{
"$match": {
"dateFieldYear": {"$eq": {"$add": [2016, 0]}}
}
}
])
--> No matches
Wait.. something is really wrong here.. Let's see with a static value:
db.testexample.aggregate([
{
"$addFields": {
"dateFieldYear": {"$year": "$dateField"}
}
},
{
"$match": {
"dateFieldYear": 2016
}
}
])
--> 1 record found!
So my conclusion seems to be that $match cannot take an expression on a field in an aggregate pipeline. But this doesn't seem possible - as the documentation states that $match follows the query syntax as described here.
Anybody can help how it can be done to $match using the simple example "dateFieldYear": {"$eq": "$dateFieldYear"}} - why doesn't this work as expected?
Thanks so much for any help

Answer 1

You can use $expr ( 3.6 mongo version operator ) to use aggregation functions in regular query.
Compare query operators vs aggregation comparison operators.
In your case
db.testexample.find({$expr:{$eq:["$dateFieldYear", "$dateFieldYear"]}})
Regular Query:
db.testexample.find({$expr:{$eq:["$dateFieldYear", {"$year": "$dateField"}]}})
Aggregation Query:
db.testexample.aggregate({$match:{$expr:{$eq:["$dateFieldYear", {"$year": "$dateField"}]}})