I want to derive `TimeCondition` entity from `Condition` entity, so I created this model (with abstract `Condition`):

![enter image description here](https://i.stack.imgur.com/P6KDf.png)

but on validation, it says:

> 
> Error 3 Error 159: EntityType 'SHomeModel.Store.TimeCondition' has no key defined. Define the key for this EntityType. E:\proj\SmartHome.edmx 126
> 
> 
> 

How can I define `TimeCondition`'s key to be `Condition`'s `Id` ?