What is the process for adding Groups to an existing type in Kontent Management API v2 with C#?

Question

If I simply attempt to add a Group using the Kontent Management API (v2) to an existing Kontent type I get the following error (see code below which generates this error):

Validation errors: 511 Every element should have a correct content group reference when using groups

What is the process for adding a group via the Management API in this case using C#? I would assume that I need to add a group Reference to all the existing elements first, but how do I do that when I cannon add the group through the API? Can I create a valid Reference simply using a new ContentGroupModel() object before adding it to the actual type in Kontent?

Here is my existing code, which throws the above error:

var updatedType = await service.ModifyContentTypeAsync(
    Reference.ById(existingContentType.Id),
    new ContentTypeAddIntoPatchModel
    {
        Path = "/content_groups",
        Value = new ContentGroupModel
        {
            Name = "New Group",
            CodeName = "new_group"
        }
    }
);

Answer 1

I was able to work it out. It turns out you can use the Codename of the new group, even before adding it in Kontent, as a reference. Then give that reference to the content_group property/path for existing elements in the model.

Here is the code I have now that adds the new Group and adds it to all existing elements dynamically:

using Kentico.Kontent.Management.Models.Shared;
using Kentico.Kontent.Management.Models.Types;
using Kentico.Kontent.Management.Models.Types.Patch;

//
// ...
//

// First, Get type from the client. Then ...
var elementCodenames = myType.Elements.Select(el => el.Codename);

// Create reference to the codename of the group to be created
var groupRef = Reference.ByCodename("new_group");

// Create the modify models
var modifyModels = new ContentTypeOperationBaseModel[] {
    new ContentTypeAddIntoPatchModel
    {
        Path = "/content_groups", // Add to content_groups path
        Value = new ContentGroupModel
        {
            Name = "New Group",
            CodeName = "new_group" // Same codename as above
        }
    }
}
.Concat(
    // Dynamically add new group, by ref, to existing elements
    elementCodenames.Select(codename => new ContentTypeReplacePatchModel
    {
        Path = $"/elements/codename:{codename}/content_group",
        Value = groupRef // Group reference created above from the codename
    })
);

// Make call to add new group AND link group to elements in the one call
var response = await client.ModifyContentTypeAsync(myTypeIdentifier, modifyModels.ToArray());

Leaving out other details, like retrieving the existing type, but for reference here are the API Docs: https://docs.kontent.ai/reference/management-api-v2