Patching a single property on a subdocument in CosmosDB

Question

I'm trying to do a partial update of a subdocument in CosmosDb. My object model is this:

public class PhoneBookCategory
{
    public string Id { get; set; }

    public string Name { get; set; }

    public TestObject SubProp { get; set; }
}

public class TestObject
{
    public string StringProp { get; set; }

    public int? IntProp { get; set; }
}

Say I want to update the Name property and SubProp, I'd do this

List<PatchOperation> updates = [];
updates.Add(PatchOperation.Add($"/Name", "new name"));
updates.Add(PatchOperation.Add($"/SubProp", new TestObject { StringProp = "hello"}));
database.GetContainer("collectionName").PatchItemAsync<PhoneBookCategory>(item.Id, new PartitionKey(item.Id), patchOperations)

This works fine. Now let's assume I want to only modify SubProp.StringProp. so, my second PatchOperation would become

updates.Add(PatchOperation.Add($"/SubProp/StringProp", "hello"));

But that just errors out with Bad Request and no useful error message (reason is ()).

I figured 'maybe, it wants a Replace for a subprop instead since in this case we know that SubProp is present, and we just want to change a value on the SubProp document, so I tried this:

updates.Add(PatchOperation.Replace($"/SubProp/StringProp", "hello"));

still, no dice.

So what am I doing wrong? According to Overview, subdocument updates are allowed.

Note that I'm using CosmosClientOptions.SerializerOptions.PropertyNamingPolicy = CosmosPropertyNamingPolicy.CamelCase (in case anyone wonders why I write the patch operations using CamelCase.

Answer 1

Below is a working sample I tested and can display detail error if you still have.

using Microsoft.Azure.Cosmos;
using Newtonsoft.Json;
internal class Program
{
    private static async Task Main(string[] args)
    {
        var endpoint = "xxx";
        var key = "xxx";
        var itemId = "a835a3c9-fa0a-42f8-b7c6-8d0e93a2e96b";
        try
        {
            var cosmosClient = new CosmosClient(endpoint,key);
            var database = cosmosClient.GetDatabase("MyDatabase");
            var container = database.GetContainer("MyContainer");

            var patchOperations = new[]
            {
                PatchOperation.Replace("/SubProp/StringProp", "hello")
            };

            ItemResponse<PhoneBookCategory> itemResponse = await container.PatchItemAsync<PhoneBookCategory>(itemId, new PartitionKey(itemId), patchOperations);
            Console.WriteLine($"Item modified with id {itemResponse.Resource.Id}");
        }
        catch (CosmosException ex)
        {
            Console.WriteLine($"Cosmos DB Exception with Status {ex.StatusCode} Message: {ex.Message}");
        }
    }
}

public class PhoneBookCategory
{
    [JsonProperty(PropertyName = "id")]
    public string Id { get; set; }
    public string Name { get; set; }
    public TestObject SubProp { get; set; }
}
public class TestObject
{
    public string StringProp { get; set; }
    public int? IntProp { get; set; }
}