Unable to aggregate multiple collections - mongodb

Question

I've user collection which has role object which has roleId. I also have roles collection which has id.
Now,for each role, I'd like to get the list of users.
Ex:
[
{
"name": "Scott",
"role": {
"roleId": "123432"
}
},
{
"name": "John",
"role": {
"roleId": "123432"
}
},
{
"name": "Scott",
"role": {
"roleId": "556432"
}
}
]
Roles Data:
[
{
"id": "123432"
"name": "admin",
"type": "internal"
},
{
"id": "556432"
"name": "owner",
"type": "external"
},
{
"id": "556432"
"name": "owner",
"type": "internal"
}
]
Now I want to get all the roles of type internal and their related users:
So, the output should be,
[
{
"role": "123432",
"users": [
{
"name": "Scott",
"role": {
"roleId": "123432"
}
},
{
"name": "John",
"role": {
"roleId": "123432"
}
}
],
{
"role": "556432",
"users": []
}
}
]
I'm doing this with Spring Boot.if someone can help me on how to get this aggregation using spring boot mongo, that'd be very helpful for me. Thank you so much.

Answer 1

Execute aggregate on your role collection and first filter out role as per your condition. Since you want all internal roles, your match stage should be:
{
$match: {
"type": "internal"
}
}
Now its time to lookup users for each role:
{
$lookup: {
from: "user",
as: "users",
localField: "id",
foreignField: "role.roleId"
}
}
I don't know how mongo queries are executed on spring boot but this is how its done on mongo so you can convert it to your language specific code.
Here's a working example on mongo playground.
https://mongoplayground.net/p/I1FPuX971YZ