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.