Deserialize flat MongoDB document to a nested C# class

Question

I am adding an API to an existing database so I am stuck with the current MongoDB document schema. The document is flat with no nested objects. I want to deserialize a group of the fields into a child object. I was wondering if there was a way of doing that with the BsonElement attribute? Another approach I came across was using a custom serializer. Would it be possible to write a custom serializer for the child object even though there is no field matching the child object's name? Or would I need to write the custom serializer at using the parent object?

Here is an example of a document:

{
  "Name": "Name",
  "AddressLine1": "321 street",
  "AddressLine2": "apt 1",
}

Here is an example of the C# classes and what I was trying to do:

public class Customer 
{
    [BsonElement("Name")]
    public string CustomerName {get; set;}

    public Address Address {get; set;}   
}

public class Address 
{
   [BsonElement("AddressLine1")]
   public string AddressLine1 {get; set;}

   [BsonElement("AddressLine2")]
   public string AddressLine2 {get; set;}
}

Answer 1

There's no way to use attributes to achieve what you are wanting, this because the attributes apply given conditions to a BsonMemberMap which resides in the BsonClassMap.

As you said this however can be achieved by creating a custom serializer.

public class CustomBsonSerializer : IBsonSerializer<MyType>
{
   
}

BsonSerializer.RegisterSerializer(new CustomBsonSerializer())

However, if you want to write a bespoke serializer it can become a bit brittle.

I'd personally opt for creating 2 sets of models, one to model the current state of your documents in MongoDB and then another model to create the structure that you want to use.