Recommended data-structure (for not querying nested data in node.js) - node.js

Question

Mongo-DB doesn't support query with wildcards in a nested structure.
In a data-structure that looks like this:
Courses = [{
'name': 'Learning node.js in 1 day',
'roles': {
'team': { subscribed: [ 'User1' ] },
'participant': { subscribed: [ 'User1', 'User2' ] },
'host': { optional: true, subscribed: [] }
}
}]
We would need wildcard-lookup to find subscribers in different roles in order not to use a query like this:
{ $or : [
{"roles.team.subscribed": 'User1'},
{"roles.participant.subscribed": 'User1'}
{"roles.host.subscribed": 'User1'}
]}
Anyway this does not work if we have an open list of roles.
And if we change it to something like this:
'roles': ['team', 'participant', 'host'],
'subscribed': [
{'user':'User1', 'roles': ['team', 'participant']},
{'user':'User2', 'roles': ['participant']}
]
it gets similarly difficult to then find all participants of a course. Either way we have a problem to find all courses a user is subscribed to.
We think about creating a separate collection for subscriptions (back to relational):
{user_id: 'User1', course_id: '456', role: 'participant'}
{user_id: 'User1', course_id: '456', role: 'team'}
{user_id: 'User2', course_id: '456', role: 'participant'}
What is best practice?
We would like to be able to make all different sorts of queries and it seems difficult if it's buried in a nested structure...
Think this is quiet a fundamental question for data-stuctures in mongoDB.

Answer 1

If you don't want to change your structure I think I would do something like this using aggregation framework. In case your fields inside roles are not so many and not completely manageable by users, in that case I would suggest you to use an array in roles and unwind it.
Let's take your first structure:
Courses = [{
'name': 'Learning node.js in 1 day',
'roles': {
'team': { subscribed: [ 'User1' ] },
'participant': { subscribed: [ 'User1', 'User2' ] },
'host': { optional: true, subscribed: [] }
}
}]
Project fields in this way:
name, roles.team, roles.participant, roles.host
So you will end up with this structure
Courses = [{
'name': 'Learning node.js in 1 day',
'roles.team.subscribed': [ 'User1' ] ,
'roles.participant.subscribed': [ 'User1', 'User2' ] ,
'roles.host.subscribed': subscribed: []
}
}]
Then you can unwind subscribed field and get a cartesian product by subscribed, you need to unwind 3 times:
Courses = [
{
'name': 'Learning node.js in 1 day',
'roles.team.subscribed.values': 'User1' ,
'roles.participant.subscribed.values': 'User1',
'roles.host.subscribed.values': ''
}
},
{
'name': 'Learning node.js in 1 day',
'roles.team.subscribed.values': 'User1' ,
'roles.participant.subscribed.values': 'User2',
'roles.host.subscribed.values': ''
}
}
]
At the end you can match every role separately.
Of course you need to use aggregation framework, takes time to get use to it.