Aggregate with lookup and contcat Strings in MongoDb - mongodb

Question

I have two collections events & members :
events Schema :
{
name : String,
members: [{status : Number, memberId : {type: Schema.Types.ObjectId, ref: 'members'}]
}
events Sample Doc :
"_id" : ObjectId("5e8b0bac041a913bc608d69d")
"members" : [
{
"status" : 4,
"_id" : ObjectId("5e8b0bac041a913bc608d69e"),
"memberId" : ObjectId("5e7dbf5b257e6b18a62f2da9"),
"date" : ISODate("2020-04-06T10:59:56.997Z")
},
{
"status" : 1,
"_id" : ObjectId("5e8b0bf2041a913bc608d6a3"),
"memberId" : ObjectId("5e7e2f048f80b46d786bfd67"),
"date" : ISODate("2020-04-06T11:01:06.463Z")
}
],
members Schema :
{
firstname : String
photo : String
}
members Sample Doc :
[{
"_id" : ObjectId("5e7dbf5b257e6b18a62f2da9"),
"firstname" : "raed",
"photo" : "/users/5e7dbf5b257e6b18a62f2da9/profile/profile-02b13aef6e.png"
},
{
"_id" : ObjectId("5e7e2f048f80b46d786bfd67"),
"firstname" : "sarra",
"photo" : "/5e7e2f048f80b46d786bfd67/profile/profile-c79f91aa2e.png"
}]
I made a query with aggregate, and lookup to get populated data of members, and I want to concat the photo fields of the members by a string, but I get an error,
How can I do the concat ?
Query :
db.getCollection('events').aggregate([
{ $match: { _id: ObjectId("5e8b0bac041a913bc608d69d")}},
{
"$lookup": {
"from": "members",
"localField": "members.memberId",
"foreignField": "_id",
"as": "Members"
}
},
{
$project: {
"Members.firstname" : 1,
"Members.photo": 1,
//"Members.photo": {$concat:["http://myurl", "$Members.photo"]},
"Members._id" : 1,
},
}
])
Result without the concat :
{
"_id" : ObjectId("5e8b0bac041a913bc608d69d"),
"Members" : [
{
"_id" : ObjectId("5e7dbf5b257e6b18a62f2da9"),
"firstname" : "raed",
"photo" : "/users/5e7dbf5b257e6b18a62f2da9/profile/profile-02b13aef6e.png"
},
{
"_id" : ObjectId("5e7e2f048f80b46d786bfd67"),
"firstname" : "sarra",
"photo" : "/5e7e2f048f80b46d786bfd67/profile/profile-c79f91aa2e.png"
}
]
}
Error :
$concat only supports strings, not array

Answer 1

You can do that simply by adding pipeline to $lookup stage
db.events.aggregate([
{
$match: {
_id: ObjectId("5e8b0bac041a913bc608d69d"),
},
},
{
$lookup: {
from: "members",
let: { memberId: "$members.memberId" },
pipeline: [
{ $match: { $expr: { $in: ["$_id", "$$memberId"] } } },
{
$project: {
firstname: 1,
photo: { $concat: ["http://myurl", "$photo"] }
}
}
],
as: "Members",
}
},
/** Optional */
{$project : {Members: 1}}
]);
Test : MongoDB-Playground

Answer 2

the alternative of using a pipeline in the above answer
we may use project and group
db.events.aggregate([
{
$match: { _id: ObjectId("5e8b0bac041a913bc608d69d") }
},
{
$unwind: '$members' // to spread the members array into a stream of documents
},
{
$lookup: {
from: "members",
localField: "members.memberId",
foreignField: "_id",
as: "member"
}
},
{
$unwind: '$member' // each document will have array of only one member, so do this unwind to convert it to an object
},
{
$project: { // do the project here to be able to use the $concat operator
'member._id': 1,
'member.firstname': 1,
'member.photo': 1,
'member.photo': { $concat: ['http://myurl', '$member.photo'] } // now we can use the $concat as member is an object, then member.photo exists
}
},
{
$group: { // do that grouping stage to gather all the members belong to the same document in one array again
_id: '$_id',
Members: {
$addToSet: '$member'
}
}
}
])