I’m using EF Core with CosmosDb, and I’m a bit confused about the change tracking and an error I’m getting when trying to save an entity.

My code looks like this:

ChatUser user = new ChatUser("example");

var response = new ChatMessage()
{
    AttachedToEntityId = attachedToEntityId,
    CustomerId = user.CustomerId,
    FromAiService = true,
    FromUser = new ChatUser(isChatBot: true),
    ToUser = user,
    Content = html
};

dbContext.ChatLog.Add(chatMessage);
await dbContext.SaveChangesAsync(ct);

This causes a strange error about the ToUser property and primary key. I have to change the ToUser assignment to:

ToUser = new ChatUser(user),

where I have a constructor that copies all the properties to the new instance.

The error is:

System.InvalidOperationException: ‘Unable to track an entity of type ‘ChatMessage.ToUser#ChatUser’ because its primary key property ‘ChatMessageAttachedToEntityId’ is null.’

I can see it’s related to the change tracking and it not attaching one entity to another, but in reality I just want it to treat the object graph as a whole and save it, not track these properties separately.

Can anyone explain the error, and is there a way to do what I was originally trying to do without needing to cloning the ‘user’ to a new instance? I’ve tried marking ChatUser with [ComplexType] but it doesn’t like the ToUser property being nullable if I do that.