I would modify detached entity with associated collection, ex :
Person and Address are POCO.
When I attach entity and save changes, the collection changes isn't detected, how can I update Person with Address (added and deleted items) ?
Do I to track my collection manually ?
Edit
The synchronization of detached POCO must be manual... EF doesn't purpose merge solution of collection (navigation properties and relations) :(
I compare the current and original collections and I detect the differences
If you are using Entity Framework, which I assume you are since you listed it as a tag on your question, then objects only track their changes when they are generated by the entity context.
That code will detect changes and persist them.
Creating the user this way will not allow the EF context to detect the changes. Instead you will need to load the entity from the database, update it, and then persist the changes.
This will allow you to pull in an entity, update it, and save the changes.