Concatenate array elements in MongoDB 3.4 with Aggregation Framework - mongodb

Question

What I have :
{ "_id" : ObjectId("577dc9d61a0b7e0a40499f90"), "equ" : 123456, "key" : "p" }
{ "_id" : ObjectId("577c789b1a0b7e0a403f1b52"), "equ" : 123456, "key" : "r" }
{ "_id" : ObjectId("577b27481a0b7e0a4033965a"), "equ" : 123456, "key" : "r" }
{ "_id" : ObjectId("5779d6111a0b7e0a40282dc7"), "equ" : 123456, "key" : "o" }
What I want :
{ "_id" : ObjectId("5779d6111a0b7e0a40282dc7"), "equ" : 123456, "keys" : "prro" }
What I tried :
db.table.aggregate([{"$group":{"_id":0, "keys":{"$push":"$key"}}}])
returns an array and not a string:
{"_id":0, "keys":["p","r","r","o"]}
Do you have any idea?

Answer 1

TL; DR
Use this aggregation pipeline:
db.col.aggregate([
{$group: {_id: "$equ", last: {$last: "$_id"}, keys: {$push: "$key"}}},
{
$project: {
equ: "$_id",
_id: "$last",
keys: {
$reduce: {
input: "$keys",
initialValue: "",
in: {$concat: ["$$value", "$$this"]}
}
}
}
}
])
More details
First you should group the documents based on the equ value and also maintain an array of keys along with the _id of the last group member:
var groupByEqu = {
$group: {
_id: "$equ",
last: {$last: "$_id"},
keys: {$push: "$key"}
}
}
Just applying this pipeline operation would result in:
{
"_id" : 123456,
"last" : ObjectId("5779d6111a0b7e0a40282dc7"),
"keys" : [ "p", "r", "r", "o" ]
}
You should use a Projection to transform the preceding document into your desired one. The first two transformations are trivial. For joining the array elements you can use the new $reduce operator:
var project = {
$project: {
equ: "$_id",
_id: "$last",
keys: {
$reduce: {
input: "$keys",
initialValue: "",
in: {$concat: ["$$value", "$$this"]}
}
}
}
}
Applying these two pipeline operations would give you the desired result:
db.col.aggregate([groupByEqu, project])
Which is:
{
"equ" : 123456,
"_id" : ObjectId("5779d6111a0b7e0a40282dc7"),
"keys" : "prro"
}