How to use group by in mongodb? - mongodb

Question

I want to values in comma seperated.So how to use group/concat in aggregate framework?
Document contact
{
"_id" : "9c612c95054fb46e3de8dee8",
"name" : "name1",
"oppID" : [
"5c612c95054fb46e3de8bcc5",
"5bd6b334cba7d2241a3ba9d9"
]
},
{
"_id" : "1c612c95054fb46e3de8dcde9",
"name" : "name2",
"oppID" : [
"5c612c95054fb46e3de8bcc5",
"5bd6b334cba7d2241a3ba9d9"
]
}
Document opportunity
{
"_id" : "5c612c95054fb46e3de8bcc5",
"name" : "opp name 01",
},
{
"_id" : "5bd6b334cba7d2241a3ba9d9",
"name" : "opp name 02",
},
I want to get contact list like below,
{
"_id" : "9c612c95054fb46e3de8dee8",
"oppName" : "opp name 01, opp name 02"
},
{
"_id" : "1c612c95054fb46e3de8dcde9",
"oppName" : "opp name 01, opp name 02"
}

Answer 1

You need to run $lookup to get the data from both collections and then $reduce with $concat to transform an array into single string
db.contact.aggregate([
{
$lookup: {
from: "opportunity",
localField: "oppID",
foreignField: "_id",
as: "opportunities"
}
},
{
$project: {
_id: 1,
oppName: {
$reduce: {
input: "$opportunities",
initialValue: "",
in: {
$cond: [ { $eq: [ "$$value", "" ] }, "$$this.name", { $concat: [ "$$value", ", ", "$$this.name" ] } ]
}
}
}
}
}
])