Mongodb Aggregation orderless items in Group _id - mongodb

Question

I am storing messages between users in a collection with this schema:
{
"_id" : ObjectId("5b23c455e3fce278f9e8d05f"), //message id
"f" : ObjectId("5ad13aaa1ba073601cc16bca"), //sender userid
"c" : "Hi", //contents
"t" : ObjectId("5ad2de5c691a4008cf6923b4"), //reciever userid
}
I am trying to query db to generate a list of current user conversations just like whatsapp list with the last message embedded using this aggregation:
db.getCollection('message').aggregate(
[
{ $match: { $or: [ { f: ObjectId("5ad13aaa1ba073601cc16bca") }, {t:ObjectId("5ad13aaa1ba073601cc16bca")} ] } },
{
$group : {
_id :{f:"$f",t:"$t"},
c: { $push: "$$ROOT" }
}
}
]
)
Result is:
{
"_id" : {
"f" : ObjectId("5ad13aaa1ba073601cc16bca"),
"t" : ObjectId("5ad2de5c691a4008cf6923b4")
},
"c" : [
{
"_id" : ObjectId("5b23c455e3fce278f9e8d05f"),
"f" : ObjectId("5ad13aaa1ba073601cc16bca"),
"c" : "Hi",
"t" : ObjectId("5ad2de5c691a4008cf6923b4"),
"d" : ISODate("2018-06-15T13:48:34.000Z"),
}
]
},
{
"_id" : {
"f" : ObjectId("5ad2de5c691a4008cf6923b4"),
"t" : ObjectId("5ad13aaa1ba073601cc16bca")
},
"c" : [
{
"_id" : ObjectId("5b235fea43966a767d2d9604"),
"f" : ObjectId("5ad2de5c691a4008cf6923b4"),
"c" : "Hello",
"t" : ObjectId("5ad13aaa1ba073601cc16bca"),
"d" : ISODate("2018-06-15T06:40:07.000Z"),
}
]
}
As you can see, there is a conversation between 5ad13aaa1ba073601cc16bca and 5ad2de5c691a4008cf6923b4. The group acts on f and t with their order. But we do just need to find conversations regardless of the order of f and t. Thus, the result document should be just like this with the latest message embedded:
{
"_id" : {
"x" : ObjectId("5ad13aaa1ba073601cc16bca"),
"y" : ObjectId("5ad2de5c691a4008cf6923b4")
},
"c" : [
{
"_id" : ObjectId("5b23c455e3fce278f9e8d05f"),
"f" : ObjectId("5ad13aaa1ba073601cc16bca"),
"c" : "Hi",
"t" : ObjectId("5ad2de5c691a4008cf6923b4"),
"d" : ISODate("2018-06-15T13:48:34.000Z"),
}
]
}
How can I handle this with aggregation? Any suggestions? Thanks.

Answer 1

Of course, you can handle it with an aggregation! As your _ids are ObjectIds, you can compare them with relational operators, therefore you can use $max and $min on them:
db.getCollection('message').aggregate([
{$match: {$or: [{f: _id}, {t: _id}]}},
{$group: {
_id: {
x: {$max: ['$f', '$t']},
y: {$min: ['$f', '$t']}
},
c: {$push: '$$ROOT'}}
}
])