Flutter Firestore understanding joins - firebase

Question

I am doing this in Flutter, with a Firebase database, using the Firestore packages in Flutter. I would like to know how a join is done in noSQL (or more specifically, in Flutter-Firestore)
This is what my database looks like:
users: {
U1: {
name: 'Peter',
surname: 'Jensen'
},
U2: {
name: 'Marry',
surname: 'Kown'
},
...
}
groups: {
G1: {
name: 'Group 1'
},
G2: {
name: 'Group 2'
},
...
}
members: {
M1: {
userId: U1,
groupId: G1
},
M2: {
userId: U1,
groupId: G2
},
M3: {
userId: U2,
groupId: G1
},
...
}
Now how do I do a join to get something like this:
members: {
M1: {
userId: {
name: 'Peter',
surname: 'Jensen'
},
groupId: {
name: 'Group 1'
}
},
M2: {
userId: {
name: 'Peter',
surname: 'Jensen'
},
groupId: {
name: 'Group 2'
}
},
M3: {
userId: {
name: 'Marry',
surname: 'Kown'
},
groupId: {
name: 'Group 1'
}
},
...
}
Do I do:
const users = await Firestore.instance.collection('users').getDocuments();
const groups = await Firestore.instance.collection('groups').getDocuments();
const members = await Firestore.instance.collection('members').getDocuments();
...manually loop through everything and assign everything myself
(I need to add more text because I have 'mostly code'): I would assume the above would use of a lot of query data in Firebase, so I can't see how this would be a good idea. I actually just want to see in what groups is a user a in

Answer 1

If you have groups and members, I'd typically store the follow data:
A list of users in a users collection.
For each user I'd keep their properties and a list of the group IDs of the groups they're a member of.
A list of groups in a groups collection.
For each group I'd keep their properties
Note that you could model the nested list as a subcollection, but typically this is not needed. With the above model you can easily find what groups a user is part of, even even do a query for users who are part of a certain group with an array-contains clause.
To get a list of the properties of the group for a specific user, you'll indeed need to load that user, and their groups separately. This is normal with many NoSQL databases, and not necessarily as slow as you may expect. But if performance is not good enough, you can consider duplicating some data to reduce the need for joins. It all depends on your needs, and unlike in relational data models, NoSQL is not dogmatic about such things.
To learn more about this topic:
read NoSQL data modeling
watch Getting to know Cloud Firestore