in mongo, cannot perform $setIntersection on array with aggregate into project - mongodb

Question

I have this two collection which each document presents a chat session:
{
"_id" : ObjectId("58136ba83bdddd2d3cd4b3fb"),
"Created" : ISODate("2016-10-28T15:15:52.563Z"),
"Messages" : [
{
"Created" : ISODate("2016-10-28T15:15:52.567Z"),
"ReadBy" : [
ObjectId("57c96a14870ae36ede4d8085"),
ObjectId("57c972b6fc8effecde6cf0fa")
],
"Content" : "Hello"
}
]
}
{
"_id" : ObjectId("5813ac380a45415df8e7fc08"),
"Created" : ISODate("2016-10-28T15:16:52.563Z"),
"Messages" : [
{
"Created" : ISODate("2016-10-28T15:15:52.567Z"),
"ReadBy" : [
ObjectId("57c96a14870ae36ede4d8234"),
ObjectId("57c972b6fc8effecde6cf987")
],
"Content" : "Hello2"
}
]
}
I am trying to use $aggregate with $project, and to perform a simple $setIntersection in order to get matching ObjectIds.
This is the projection:
db.getCollection('Chats').aggregate([
{
$project: {
Created: 1,
Messages: 1,
Match: {
$setIntersection: [ "$Messages.ReadBy", [ObjectId("57c96a14870ae36ede4d8085")] ]
},
ReadBy: "$Messages.ReadBy"
}
}
])
The result i am getting from this aggregate, seems to insert "$Messages.ReadBy" into Sub-Array of Array [[ObjectId("....")]] for example.
The Result is that $setIntersection returns null although there is something relevant for me, and the new "ReadBy" field i added for debug shows the problem of sub-array:
{
"_id" : ObjectId("58136ba83bdddd2d3cd4b3fb"),
"Created" : ISODate("2016-10-28T15:15:52.563Z"),
"Messages" : [
{
"Created" : ISODate("2016-10-28T15:15:52.567Z"),
"ReadBy" : [
ObjectId("57c96a14870ae36ede4d8085"),
ObjectId("57c972b6fc8effecde6cf0fa")
],
"Content" : "Hello"
}
],
"Match" : [],
"ReadBy" : [
[
ObjectId("57c96a14870ae36ede4d8085"),
ObjectId("57c972b6fc8effecde6cf0fa")
]
]
}
{
"_id" : ObjectId("5813ac380a45415df8e7fc08"),
"Created" : ISODate("2016-10-28T15:16:52.563Z"),
"Messages" : [
{
"Created" : ISODate("2016-10-28T15:15:52.567Z"),
"ReadBy" : [
ObjectId("57c96a14870ae36ede4d8234"),
ObjectId("57c972b6fc8effecde6cf987")
],
"Content" : "Hello2"
}
],
"Match" : [],
"ReadBy" : [
[
ObjectId("57c96a14870ae36ede4d8234"),
ObjectId("57c972b6fc8effecde6cf987")
]
]
}
Why is "Match" field returns empty array?
And why "ReadBy" wrapping the source field with extra array?

Answer 1

a solution is to use $arrayElemAt to get the sub array.
the new query is
db.getCollection('Chats').aggregate([
{
$project:{
Created:1,
Messages:1,
Match:{
$setIntersection:[
{
$arrayElemAt:[
"$Messages.ReadBy",
0
]
},
[
ObjectId("57c96a14870ae36ede4d8085")
]
]
},
ReadBy:"$Messages.ReadBy"
}
}
])
and the result is now
{
"_id" : ObjectId("58136ba83bdddd2d3cd4b3fb"),
"Created" : ISODate("2016-10-28T15:15:52.563Z"),
"Messages" : [
{
"Created" : ISODate("2016-10-28T15:15:52.567Z"),
"ReadBy" : [
ObjectId("57c96a14870ae36ede4d8085"),
ObjectId("57c972b6fc8effecde6cf0fa")
],
"Content" : "Hello"
}
],
"Match" : [
ObjectId("57c96a14870ae36ede4d8085")
],
"ReadBy" : [
[
ObjectId("57c96a14870ae36ede4d8085"),
ObjectId("57c972b6fc8effecde6cf0fa")
]
]
}
{
"_id" : ObjectId("5813ac380a45415df8e7fc08"),
"Created" : ISODate("2016-10-28T15:16:52.563Z"),
"Messages" : [
{
"Created" : ISODate("2016-10-28T15:15:52.567Z"),
"ReadBy" : [
ObjectId("57c96a14870ae36ede4d8234"),
ObjectId("57c972b6fc8effecde6cf987")
],
"Content" : "Hello2"
}
],
"Match" : [ ],
"ReadBy" : [
[
ObjectId("57c96a14870ae36ede4d8234"),
ObjectId("57c972b6fc8effecde6cf987")
]
]
}