Cosmos DB: save object type as a property in document

Question

I want to save object type as a part of json object when upsert my object into cosmos db. I've tried to pass a json serializer when instantiating instance of Cosmos Client, but it doesn't work. I still don't see type of object in document. What I was trying to do:

    public static readonly JsonSerializerSettings DefaultJsonSerializerSettings =
        new JsonSerializerSettings
        {
            TypeNameHandling = TypeNameHandling.All,
            DateFormatString = "o",
            DateFormatHandling = DateFormatHandling.IsoDateFormat,
        }; 

    var CosmosClient =
            new DocumentClient(
                new Uri(CosmosConfig.ServiceEndpoint),
                CosmosConfig.AuthNKey,
                DefaultJsonSerializerSettings,
                connectionPolicySettings);

Any other way to have such a behavior without preprocessing (converting object to jObject) first? Thanks

Upd:

What I trying to achive is smth like next structure in my document (automaticaly serialized type):

    {
      "$type" : "MyNamespace.Foo",
      "Id": "1560e1be-bf87-4720-a22e-b7e2c4c37f2e",
      "Name" : "Vasia"
    }

instead of current one like this (without type):

    {
      "Id": "1560e1be-bf87-4720-a22e-b7e2c4c37f2e",
      "Name" : "Vasia"
    }

HenriM HenriM · Accepted Answer · 2018-05-31T15:50:57

When using Cosmos DB I often let my documents/classes inherit from an abstract base class similar to this:

public abstract class DocModel
{
    [JsonProperty(PropertyName = "$type")]
    public virtual string Doctype => GetType().Name;

    [JsonProperty(PropertyName = "id")]
    public string Id { get; set; }
}

which pretty much give you what you were looking for:

public class Cloud : DocModel
{
    public string Name { get; set; }
}

public class Foo : DocModel
{
    public string Name { get; set; }
}

Would end up as

{
   "$type":"Cloud",
   "id": "1560e1be-bf87-4720-a22e-b7e2c4c37f2e",
   "name" : "Vasia"
}

{
   "$type":"Foo",
   "id": "2560e1be-bf87-4720-a22e-b7e2c4c37f2e",
   "name" : "Vasia2"
}

You could change Doctype property to GetType().FullName etc. to get namespace and such.

This also allows you to query all documents based on Doctype, something like:

   var t = typeof(T).Name;
   IDocumentQuery<T> query = _db.Client
            .CreateDocumentQuery<T>(_db.CollectionUri, new FeedOptions { MaxItemCount = -1 })
            .Where(predicate)
            .Where(_ => _.Doctype == t)
            .AsDocumentQuery();

To be used in a generic repository for example.

Cosmos DB: save object type as a property in document

2 Answers