Get the most recent document in a Cosmos DB container using IQueryable - azure-cosmosdb

Question

I need to get the most recent document in a Cosmos DB container using an IQueryable query.
The SQL query corresponding to what I want to do is:
SELECT top 1 * FROM c order by c._ts
What I have available is an IQueryable provided by a generic repository. The repository creates the IQueryable like this:
var queryable = container.GetItemLinqQueryable<TEntity>(false, null, requestOption);
And the Entity (in IQueryable) does NOT contain a field that maps to "_ts":
public class Entity {
public string id { get; set; }
public Name { get; set; }
}
Here is the code that I need to write, but there is a missing timestamp field:
var queryable = await _repo.GetAsync(partitionKey);
var query = queryable.OrderByDescending(e => e.{The Missing _ts field}).Take(1);
Is there a way to write such a LINQ query?
Note: I am using .NET Cosmos DB SDK (v3).

Answer 1

Add the ts attribute on your Entity class something like below. Now you can query on Timestamp.
[Newtonsoft.Json.JsonConverter(typeof(Microsoft.Azure.Documents.UnixDateTimeConverter))]
[Newtonsoft.Json.JsonProperty(PropertyName="_ts")]
private virtual DateTime Timestamp { get; }

Answer 2

Adding _ts attribute plainly throws error when ingesting the data without setting the value for TimeStamp field(Default value is "01/01/0001" for DateTime). It works fine when fetching data, but when ingesting we will run into issues. Solution is to use a nullable type which will work both for insertion, retrieval.
[Newtonsoft.Json.JsonConverter(typeof(Microsoft.Azure.Documents.UnixDateTimeConverter))]
[Newtonsoft.Json.JsonProperty(NullValueHandling=Newtonsoft.Json.NullValueHandling.Ignore, PropertyName="_ts")]
public DateTime? LastModified { get; }
The key here is NullValueHandling.Ignore and nullable type (DataTime?). If we have these values, Cosmos will internally update the _ts field when we insert or merge. So, no need to set the value in our code during ingestion. Querying for documents will fetch the value as usual.
The above will cause an issue if you try to fetch an element and update some properties of that element. Now if you try to push this element to cosmos, it will not update the latest modified time because we already have the value set in _ts.
To overcome this, add shouldserialize method.
private bool ShouldSerializeLastModified()
{
// never update last modified timestamp when updating entries in cosmos. Let cosmos do that for us.
return false;
}
Reference: https://learn.microsoft.com/en-us/dotnet/api/microsoft.azure.cosmos.databaseproperties.lastmodified?view=azure-dotnet
https://learn.microsoft.com/en-us/dotnet/desktop/winforms/controls/defining-default-values-with-the-shouldserialize-and-reset-methods?view=netframeworkdesktop-4.8