Advanced Databases

How to organize data? remember:

What becomes a dedicated collection? What becomes nested document?

In MongoDB, model implementation should translate data usage
It means that denormalization (redundancy) can be acceptable if it optimizes the way data are consumed
Some limits :
- It is recommended to not have big nested lists (through document field), in that case, use references
- Avoid deep nesting of documents, it can cause performances issues

Possible solution for the exercise

Entities that can form long lists will be stored by default in collections
We can nest other entities, inheritance disappear from implementation, "thanks to" schema less capabilities of Mongo
→ In that last case, variants of the main domain entity have optional attributes

solution for physical model implementation

How to perform "joins" across collections? 1st, store the reference

It is possible to create a reference from the unique identifier of an other document

Example :

db.answers.insertOne({
    question: ObjectId('...'),
    student : ObjectId('...'),
    exam: ObjectId('...'),
    choices :[1,2]
})

Note: reference can be made by whatever field, not necessarily by the id fields

How to perform "joins" across collections? 2nd: use $lookup

The $lookup operator allows to resolve references as nested documents in the results Example :
```
db.answers.aggregate({
    $lookup: {
        from: "questions",
        localField: "question",
        foreignField: "_id",
        as: "questionData"
    }
})
        
```
will resolve all the references coming from the questions collection and place the reference in a field questionData

Starting implementation

Create the collections and define the documents
Prepare the code to operate the collections
Put all the instructions in a js file
Call those queries from the console or from Compass
Call the same queries from your preferred programming language

Basic operations for questions : create

//create
db.questions.insertOne({
    choices:[
        {
            choice:1,
            choiceLabel:"abc",
            valid:true
        }
    ],
    difficulty: 0,
    subject:"",
    topics:[]
});

Basic operations for questions : delete

db.questions.deleteOne({
    _id: ObjectId('')
});

Note: you can put whatever criteria you want instead of id, be sure then to target only the documents you really want to delete, that's quite dangerous

Basic operations for questions : update

//updateById
db.questions.updateOne(
    {_id:ObjectId('')},
    {$set: {subject:''}}
)

Note: you can put whatever criteria you want instead of id, be sure then to target only the documents you really want to update

Basic operations for questions : find

//search by topics
db.questions.find({
    topics: ['']
})