I have been using MongoDB for few months. It is one of the arrays of open source NoSQL solutions available. The way MongoDB including CouchDB stores data in JSON format as a document (It’s called BSON, Binary JSON in MongoDB). A document is a loosely typed data format as compared to strictly typed in SQL based systems. You can find more detail information about document data format here.  In MongoDB, all those documents are contained in a collection which acts as a bucket for those documents. So, a collection “contact_info” may be formatted as:

{
    "_id": "Prajwal Tuladhar"
    "street": "3478 78 ST",
    "zip": 11378,
    "email": "praj@prajwal-tuladhar.net.np",
    "phone": {
        "home": "64622251452"
    }
}

{
    "_id": "Max Payne"
    "street": "17th Floor 225 Park Avenue",
    "zip": 10001,
    "phone": {
        "mobile": "7182524423",
        "office": "2127783264"
    },
    "fax": "1.913.384.6577"
}

So these two documents are contained in a collection called “contact_info”. _id is a mandatory field in MongoDB while others are user defined. As you can see, documents are self-containded and their structure is slighly different from each other. This is one of the main advantages of using document based systems. One can define rich data structure without sacrificing SQL like features with option to query them in number of ways.

If you look at the above two documents, field “phone” is an example of embedded document. Other examples may be:

db.students
=======

{
    "_id": "Jane",
    "address": {
        "address": "123 Main St",
        "city": "New York",
        "zip": 10011
    },
    "scores": [
        {
            "subject": "Biology",
            "grade": 4.00
        },
        {
            "subject": "Physics",
            "grade": 3.50
        }
    ]
}

While designing schemas for MongoDB, one of the issues I have faced for sometime is: when to use embedded document.

Here are some of the excerpts about embedded documents from MongoDB docs:

Line item detail objects typically are embedded.

Objects which follow an object modelling “contains” relationship should generally be embedded.

I think one key point is missing here. The document should be embedded or used as a reference depending on the context of the domain. If the document can be expressed as domain model then, it should be declared as first class document while optionally expressing their relationship as reference or it can be also done from application level rather than DB level (Just imagine table relationship for MySQL ISAM). So, in the above examples, if the scores can be expressed as domain object (which depends on the context of the application), it should not be used as embedded document.

And from database point of view also, embedded documents are hard to update / manipulate while they are loaded in advance which can be problematic from performance point of view when size is freaking high. There is no concept of lazy loading in the form of cursor object for embedded documents so, one should acknowledge that large embedded documents may lead to bad design and the maximum size of the MongoDB document is 4 MB,