MongoDB lookup when foreign field is an array - mongodb

Question

I've searched the internet and StackOverflow, but I cannot find the answer or even the question.
I have two collections, reports and users. I want my query to return all reports and indicate if the specified user has that report as a favorite in their array.
Reports Collection
{ _id: 1, name:"Report One"}
{ _id: 2, name:"Report Two"}
{ _id: 3, name:"Report Three"}
Users Collection
{_id: 1, name:"Mike", favorites: [1,3]}
{_id: 2, name:"Tim", favorites: [2,3]}
Desired Result for users.name="Mike"
{ _id: 1, name:"Report One", favorite: true}
{ _id: 2, name:"Report Two", favorite: false}
{ _id: 3, name:"Report Three", favorite: true}
All of the answers I can find use $unwind on the local (reports) field, but in this case the local field isn't an array. The foreign field is the array.
How can I unwind the foreign field? Is there a better way to do this?
I saw online that someone suggested making another collection favorites that would contain:
{ _id: 1, userId: 1, reportId: 1 }
{ _id: 2, userId: 1, reportId: 3 }
{ _id: 3, userId: 2, reportId: 2 }
{ _id: 4, userId: 2, reportId: 3 }
This method seems like it should be unnessesary. It should be simple to join onto an ID in a foreign array, right?

Answer 1

You can use $lookup with custom pipeline which will give you 0 or 1 result and then use $size to convert an array to single boolean value:
db.reports.aggregate([
{
$lookup: {
from: "users",
let: { report_id: "$_id" },
pipeline: [
{
$match: {
$expr: {
$and: [
{ $eq: [ "$name", "Mike" ] },
{ $in: [ "$$report_id", "$favorites" ] }
]
}
}
}
],
as: "users"
}
},
{
$project: {
_id: 1,
name: 1,
favorite: { $eq: [ { $size: "$users" }, 1 ] }
}
}
])
Alternatively if you need to use MongoDB version lower than 3.6 you can use regular $lookup and then use $filter to get only those users where name is Mike:
db.reports.aggregate([
{
$lookup: {
from: "users",
localField: "_id",
foreignField: "favorites",
as: "users"
}
},
{
$project: {
_id: 1,
name: 1,
favorite: { $eq: [ { $size: { $filter: { input: "$users", as: "u", cond: { $eq: [ "$$u.name", "Mike" ] } } } }, 1 ] }
}
}
])

Answer 2

"_id" : ObjectId("611fc392cfadfbba65d4f4bd"),
"t_name" : "Bahadur",
"t_age" : "22",
"trch" : "java",
"StudentsDetails" : [
{
"_id" : ObjectId("611fc41ccfadfbba65d4f4be"),
"s_name" : "Asin",
"s_age" : "18",
"trch" : "java",
"tsid" : ObjectId("611fc392cfadfbba65d4f4bd")
},
{
"_id" : ObjectId("611fc8f1a815fb2c737ae31f"),
"s_name" : "sonu",
"s_age" : "18",
"tsid" : ObjectId("611fc392cfadfbba65d4f4bd")
},
{
"_id" : ObjectId("611fc915a815fb2c737ae320"),
"s_name" : "monu",
"s_age" : "19",
"tsid" : ObjectId("611fc392cfadfbba65d4f4bd")
}
]
}
Create Trainer Collection
Create Scholar Collection
//query
db.Trainer.aggregate(
[`enter code here`
{`enter code here`
$lookup:`enter code here`
{`enter code here`
from: "scholar",`enter code here`
localField: "_id",`enter code here`
foreignField: "tsid",`enter code here`
as: "StudentsDetails"`enter code here`
}`enter code here`
}`enter code here`
]`enter code here`
).pretty();