Convert array of objects to array of strings in mongodb - mongodb

Question

I am looking at the following documentation.
The following inserts documents into a collection classes.
db.classes.insertMany( [
{ _id: 1, title: "Reading is ...", enrollmentlist: [ "giraffe2", "pandabear", "artie" ], days: ["M", "W", "F"] },
{ _id: 2, title: "But Writing ...", enrollmentlist: [ "giraffe1", "artie" ], days: ["T", "F"] }
] )
And the members collection:
db.members.insertMany( [
{ _id: 1, name: "artie", joined: new Date("2016-05-01"), status: "A" },
{ _id: 2, name: "giraffe", joined: new Date("2017-05-01"), status: "D" },
{ _id: 3, name: "giraffe1", joined: new Date("2017-10-01"), status: "A" },
{ _id: 4, name: "panda", joined: new Date("2018-10-11"), status: "A" },
{ _id: 5, name: "pandabear", joined: new Date("2018-12-01"), status: "A" },
{ _id: 6, name: "giraffe2", joined: new Date("2018-12-01"), status: "D" }
] )
They use the following aggregation to join the two collections on the array field, enrollmentlist.
db.classes.aggregate( [
{
$lookup:
{
from: "members",
localField: "enrollmentlist",
foreignField: "name",
as: "enrollee_info"
}
}
] )
Which returns the following:
{
"_id" : 1,
"title" : "Reading is ...",
"enrollmentlist" : [ "giraffe2", "pandabear", "artie" ],
"days" : [ "M", "W", "F" ],
"enrollee_info" : [
{ "_id" : 1, "name" : "artie", "joined" : ISODate("2016-05-01T00:00:00Z"), "status" : "A" },
{ "_id" : 5, "name" : "pandabear", "joined" : ISODate("2018-12-01T00:00:00Z"), "status" : "A" },
{ "_id" : 6, "name" : "giraffe2", "joined" : ISODate("2018-12-01T00:00:00Z"), "status" : "D" }
]
}
{
"_id" : 2,
"title" : "But Writing ...",
"enrollmentlist" : [ "giraffe1", "artie" ],
"days" : [ "T", "F" ],
"enrollee_info" : [
{ "_id" : 1, "name" : "artie", "joined" : ISODate("2016-05-01T00:00:00Z"), "status" : "A" },
{ "_id" : 3, "name" : "giraffe1", "joined" : ISODate("2017-10-01T00:00:00Z"), "status" : "A" }
]
}
How can I reduce enrolle_info just to be an array of strings with all of the names?
This is the result I am after:
{
"_id" : 1,
"title" : "Reading is ...",
"enrollmentlist" : [ "giraffe2", "pandabear", "artie" ],
"days" : [ "M", "W", "F" ],
"enrollee_info" : [
"artie",
"pandabear"
"giraffe2"
]
}
{
"_id" : 2,
"title" : "But Writing ...",
"enrollmentlist" : [ "giraffe1", "artie" ],
"days" : [ "T", "F" ],
"enrollee_info" : [
"artie",
"giraffe1"
]
}
I have also looked into using multiple joins by introducing the pipeline field inside the $lookup operation. I am able to use a $project to get an array just with {"name": "example"} but I am not sure how to remove the "name". I have tried using an {"$unwind": "$enrollee_info.name"} but that does not give me what I want. Do I need to introduce another stage to my aggregation pipeline after I do the join?

Answer 1

It seems that I was over complicating this. I was able to achieve my desired result by doing the following:
db.classes.aggregate( [
{
$lookup:
{
from: "members",
localField: "enrollmentlist",
foreignField: "name",
as: "enrollee_info"
}
},
{
$project:
{
"_id": 1,
"title": 1,
"days": 1,
"enrollee_names": "$enrollee_info.name"
}
}
] )
Result:
[
{
"id": 1,
"title": "Reading is ...",
"days": [
"M",
"W",
"F"
],
"names": [
"artie",
"pandabear",
"giraffe2"
]
},
{
"id": 2,
"title": "But Writing ...",
"days": [
"T",
"F"
],
"names": [
"artie",
"giraffe1"
]
}
]