I assume you have created a new Domain Model and added the Dinner and RSVP tables. If you've used the default settings then the Dinner class is in the NerdDinner namespace, not the NerdDinner.Models.
Another option is to create the Domain Model specifying a custom namespace, in this case NerdDinner.Models. Then the classes will be in the correct namespace. There is however an issue with the validation, although you will be able to build the solution successfully you will get errors about an invalid namespace.
This will be fixed in the next release.
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