join collections in mongodb [duplicate] - mongodb

Question

I coming from a sql background so writing queries in sql where I join tables is quite simple but I guess I am missing that in mongoose/mongodb
Basically I know the Subscriber_ID (which maps to a document in the User Collection)
I want to pull the project group, with all the projects that the user belongs to so if I was to write this in pseduo sql it would be like
Select
ProjectGroup.title,
Project.Title
FROM
ProjectGroup,
Project,
User
WHERE
User.id = req.body.subscriber_id
AND Project.subscriber_id = User.id
AND ProjectGroup.project_id = Project.id
There must be a way to do similiar joins in mongoose/mongodb because the type is mapping to a schema right?
My Schemas.....
Project Group Schema
var ProjectGroupSchema = new Schema({
title : String
, projects : [ { type: Schema.Types.ObjectId, ref: 'Project' } ]
});
Project Schema
var ProjectSchema = new Schema({
title : {type : String, default : '', required : true}
, subscribers : [{ type: Schema.Types.ObjectId, ref: 'User' }]
});
User Schema
var UserSchema = new Schema({
first_name : {type: String, required: true}
, last_name : {type: String, required: true}
});
Thank you!

Answer 1

You are just one step away!
Project Group Schema:
var ProjectGroupSchema = new Schema({
title : String
});
Project Schema:
var ProjectSchema = new Schema({
title : {type : String, default : '', required : true},
group : {type: Schema.Types.ObjectId, ref: 'ProjectGroup' },
_users : [{type: Schema.Types.ObjectId, ref: 'User' }]
});
User Schema:
var UserSchema = new Schema({
first_name : {type: String, required: true},
last_name : {type: String, required: true},
subscribing : [{type: Schema.Types.ObjectId, ref: 'Project' }]
});
Then you can do the following:
user.findById(req.userId)
.populate('subscribing')
.exec(function(err, user){
console.log(user.subscribing);
})
Or:
project.find({
subscriber : req.userId
})
.populate('subscriber')
.populate('group')
.exec(function(err, projects){
console.log(projects);
})

Answer 2

There are no joins in Mongodb. This question I think is a good reference:
MongoDB and "joins"
To summarize, different strategies have to be adopted with mongodb for problems that would be addressed via joins in relational DBs. I would say you mainly end-up doing one of these two things:
Embedding: You embed information in a single document that would in a relational DB be distributed amongst different tables.
Joining information client-side: When you need to use information from several places, you query many times and then put the pieces together in your client.