Mixing Static and Dynamic Data

Many applications have the notion of known data and unknown data. One such example would be a Blog application where there exists the known data (content, created date, tags) and unknown data (metadata) which a user is allowed to configure.

The Class

Continuing with the Blog example, below is how we might model the static content for a post.

public class Post
{
    public ObjectId Id { get; set; }
  
    public string Content { get; set; }
  
    public DateTime CreatedAtUtc { get; set; }
  
    public List<string> Tags { get; set; }  
}

In order to include dynamic data, we have 2 options.

BsonDocument Property

The first option is to include a BsonDocument. Our class would then look as follows:

class Post
{
    // previous properties
  
    public BsonDocument Metadata { get; set; }
}

We can then put all the dynamic data into the Metadata property.

var post = new Post
{
    Content = "My Post Content",
    CreatedAtUtc = DateTime.UtcNow,
    Tags = new List<string> { "first", "post" },
    Metadata = new BsonDocument("rel", "mongodb")
};

In the above example, we set the static properties, but also added a rel field with a value of mongodb to our metadata. Not all documents may have the rel field because it isn’t part of the schema. The above document would look like this in the database:

{
    "_id" : ObjectId("5564b6c11de315e733f173cf"),
    "Content": "My Post Content",
    "CreatedAtUtc" : ISODate("2015-05-26T18:09:05.883Z"),
    "Tags" : ["first", "post"],
    "Metadata": {
        "rel": "mongodb"   
    }
}

Using this method, all our “dynamic” data is stored underneath the Metadata field.

Extra Elements

The second option is to store dynamic data using the extra elements feature in the Bson library.

Our full class would like this:

public class Post
{
    public ObjectId Id { get; set; }
  
    public string Content { get; set; }
  
    public DateTime CreatedAtUtc { get; set; }
  
    public List<string> Tags { get; set; }  
    
    [BsonExtraElements]
    public BsonDocument Metadata { get; set; }
}

Using the following code to create Post:

var post = new Post
{
    Content = "My Post Content",
    CreatedAtUtc = DateTime.UtcNow,
    Tags = new List<string> { "first", "post" },
    Metadata = new BsonDocument("rel", "mongodb")
};

Our document in the database would look like this:

{
    "_id" : ObjectId("5564b6c11de315e733f173cf"),
    "Content": "My Post Content",
    "CreatedAtUtc" : ISODate("2015-05-26T18:09:05.883Z"),
    "Tags" : ["first", "post"],
    "rel": "mongodb"   
}

Using this method, the rel field is stored inline with the static content.