Update array of subdocuments in MongoDB

Question

I have a collection of students that have a name and an array of email addresses. A student document looks something like this:

{
  "_id": {"$oid": "56d06bb6d9f75035956fa7ba"},
  "name": "John Doe",
  "emails": [
    {
      "label": "private",
      "value": "private@johndoe.com"
    },
    {
      "label": "work",
      "value": "work@johndoe.com"
    }
  ]
}

The label in the email subdocument is set to be unique per document, so there can't be two entries with the same label.

My problems is, that when updating a student document, I want to achieve the following:

• adding an email with a new label should simply add a new subdocument with the given label and value to the array
• if adding an email with a label that already exists, the value of the existing should be set to the data of the update

For example when updating with the following data:

{
  "_id": {"$oid": "56d06bb6d9f75035956fa7ba"},
  "emails": [
    {
      "label": "private",
      "value": "me@johndoe.com"
    },
    {
      "label": "school",
      "value": "school@johndoe.com"
    }
  ]
}

I would like the result of the emails array to be:

"emails": [
    {
      "label": "private",
      "value": "me@johndoe.com"
    },
    {
      "label": "work",
      "value": "work@johndoe.com"
    },
    {
      "label": "school",
      "value": "school@johndoe.com"
    }
  ]

How can I achieve this in MongoDB (optionally using mongoose)? Is this at all possible or do I have to check the array myself in the application code?

Answer 1

After investigating the different options posted, I decided to go with my own approach of doing the update manually in the code using lodash's unionBy() function. Using express and mongoose's findById() that basically looks like this:

Student.findById(req.params.id, function(err, student) {

    if(req.body.name) student.name = req.body.name;
    if(req.body.emails && req.body.emails.length > 0) {
        student.emails = _.unionBy(req.body.emails, student.emails, 'label');
    }

    student.save(function(err, result) {
        if(err) return next(err);
        res.status(200).json(result);
    });

});

This way I get the full flexibility of partial updates for all fields. Of course you could also use findByIdAndUpdate() or other options.

Alternate approach:

However the way of changing the schema like Vince Bowdren suggested, making label a single separate field in a email subdocument, is also a viable option. In the end it just depends on your personal preferences and if you need strict validation on your data or not.

If you are using mongoose like I do, you would have to define a separate schema like so:

var EmailSchema = new mongoose.Schema({
  work: { type: String, validate: validateEmail },
  private: { type: String, validate: validateEmail }
}, {
  strict: false,
  _id: false
});

In the schema you can define properties for the labels you already want to support and add validation. By setting the strict: false option, you would allow the user to also post emails with custom labels. Note however, that these would not be validated. You would have to apply the validation manually in your application similar to the way I did it in my approach above for the merging.

Answer 2

Suggestion: refactor your data to use the "label" as an actual field name.

There is one straightforward way in which MongoDB can guarantee unique values for a given email label - by making the label a single separate field in itself, in an email sub-document. Your data needs to exist in this structure:

{
  "_id": ObjectId("56d06bb6d9f75035956fa7ba"),
  "name": "John Doe",
  "emails": {
      "private": "private@johndoe.com",
      "work" : "work@johndoe.com"
  }
}

Now, when you want to update a student's emails you can do an update like this:

db.students.update(
    {"_id": ObjectId("56d06bb6d9f75035956fa7ba")},
    {$set: {
        "emails.private" : "me@johndoe.com",
        "emails.school" : "school@johndoe.com"
    }}
);

And that will change the data to this:

{
  "_id": ObjectId("56d06bb6d9f75035956fa7ba"),
  "name": "John Doe",
  "emails": {
      "private": "me@johndoe.com",
      "work" : "work@johndoe.com",
      "school" : "school@johndoe.com"
  }
}

Admittedly there is a disadvantage to this approach: you will need to change the structure of the input data, from the emails being in an array of sub-documents to the emails being a single sub-document of single fields. But the advantage is that your data requirements are automatically met by the way that JSON objects work.

Answer 3

You could try this update but only efficient for small datasets:

mongo shell:

var data = {
    "_id": ObjectId("56d06bb6d9f75035956fa7ba"),
    "emails": [
        {
          "label": "private",
          "value": "me@johndoe.com"
        },
        {
          "label": "school",
          "value": "school@johndoe.com"
        }
    ]
};

data.emails.forEach(function(email) {
    var emails = db.students.findOne({_id: data._id}).emails,
        query = { "_id": data._id },
        update = {};

    emails.forEach(function(e) {
        if (e.label === email.label) {
            query["emails.label"] = email.label;
            update["$set"] = { "emails.$.value": email.value };
        } else {
           update["$addToSet"] = { "emails": email }; 
        }
        db.students.update(query, update)
    });
});