How to combine Documents in aggregation pipeline with MongoDB Java driver 3.6? - mongodb

Question

I am using an aggregation pipeline with the MongoDB Java driver version 3.6. If I have documents that look something like:
doc1 --
{
"CAR": {
"VIN": "ASDF1234",
"YEAR": "2018",
"MAKE": "Honda",
"MODEL": "Accord"
},
"FEATURES": [
{
"AUDIO": "MP3",
"TIRES": "All Season",
"BRAKES": "ABS"
}
]
}
doc2 --
{
"CAR": {
"VIN": "ASDF1234",
"AVAILABILITY": "In Stock"
}
}
And if I submit a query like:
collection.aggregate(
Arrays.asList(
Aggregates.match(
and(
in("CAR.VIN", vinList),
or(
eq("CAR.MAKE", carMake),
eq("CAR.AVAILABILITY", carAvailability),
)
)
)
)
)
Let us assume that there are exactly two different records for which the "CAR.VIN" criteria match for every VIN, and I am going to get two results. Rather than deal with two results each time, I would like to merge the documents so that the result looks like this:
{
"CAR": {
"VIN": "ASDF1234",
"YEAR": "2018",
"MAKE": "Honda",
"MODEL": "Accord",
"AVAILABILITY": "In Stock"
},
"FEATURES": [
{
"AUDIO": "MP3",
"TIRES": "All Season",
"BRAKES": "ABS"
}
]
}
The example where I have two and only two results trivializes my need for this. Imagine that vinList is a list of 10000 values, and it might return 2 x 10000 documents. When I return an AggregateIterable to the client that is calling my code, I do not want to impose the requirement that they have to group or collate the results in any way, but that they will receive one document for each result that has all of the information that they will want to parse, cleanly and easily.
Of course, people will suggest that the data is simply combined into one document with all of the data in the MongoDB collection. For reasons that I cannot control, there are two separate documents corresponding to each VIN in the same collection, and that is something that I am unable to change. There is a value in our system that makes this more reasonable than it might seem, so please don't focus on this apparent problem with the data.
I am trying, with not much luck, to utilize the Aggretes.group() operation to merge the fields in my aggregation pipeline. Accumulators.push seems to be the closest operation to what I need, but I do not want to complicate the document structure with extra arrays, etc. Is there a straightforward approach that I am not seeing?

Answer 1

you can try $mergeObjects added in mongo v3.6
db.cc.aggregate(
[
{
$group: {
_id : "$CAR.VIN",
CAR : {$mergeObjects : "$CAR"},
FEATURES : {$mergeObjects : {$arrayElemAt : ["$FEATURES", 0 ]}}
}
}
]
).pretty()
result
{
"_id" : "ASDF1234",
"CAR" : {
"VIN" : "ASDF1234",
"YEAR" : "2018",
"MAKE" : "Honda",
"MODEL" : "Accord",
"AVAILABILITY" : "In Stock"
},
"FEATURES" : {
"AUDIO" : "MP3",
"TIRES" : "All Season",
"BRAKES" : "ABS"
}
}
>
to get features as array
db.cc.aggregate(
[
{
$group: {
_id : "$CAR.VIN",
CAR : {$mergeObjects : "$CAR"},
FEATURES : {$push : {$arrayElemAt : ["$FEATURES", 0 ]}}
}
}
]
).pretty()
result
{
"_id" : "ASDF1234",
"CAR" : {
"VIN" : "ASDF1234",
"YEAR" : "2018",
"MAKE" : "Honda",
"MODEL" : "Accord",
"AVAILABILITY" : "In Stock"
},
"FEATURES" : [
{
"AUDIO" : "MP3",
"TIRES" : "All Season",
"BRAKES" : "ABS"
},
null
]
}
>