[eluser]WanWizard[/eluser]
You need the primary key to uniquely identify a record. In your example, there is no way to do that, so how are you going to delete or modify a single employee record?
You may think that you have defined a one-to-one (which by the way isn't a relationship that exists in Datamapper), but you can easily have multiple employee records with the same user_id (for example someone that works part-time for two departments?).
You will have to make this a one-to-one in your application code, by making sure you don't create one with a given user_id while another one exists. Datamapper is not aware of that.
For this to work, you will need a "belongs_to" type of relation, which doesn't exist in Datamapper. It will also need complete new logic for interacting with tables, making the library even more complex.
