Should Entities in Domain Driven Design and Entity Framework be the same?
I have started using Entity Framework Code First for the first time and am impressed by the way in which our greenfield application is being built around the domain rather than around the relational database tables (which is how I have worked for years).
So, we are building entities in C# that are being reflected in the database every time we do a new migration.
My question is this: should these same entities (i.e. designed with Entity Framework in mind) play the same role as entities in Domain Driven Design (i.e. representing the core of the domain)?
Answer
Object-Relational Mapping and Domain-Driven Design are two orthogonal concerns.
ORM
An ORM is just here to bridge the gap between the relational data model residing in your database and an object model, any object model.
An Entity as defined by EF concretely means any object that you wish to map some subpart of your relational model to (and from). It turns out that the EF creators wanted to give a business connotation to those by naming them Entities, but in the end nothing forces you that way. You could map to View Models for all it cares.
DDD
From a DDD perspective, there's no such thing as "an Entity designed with EF in mind". A DDD Entity should be persistence ignorant and bear no trace of any ORM. The domain layer has no interest in how, where, whether or when its objects are stored.
Where the two meet
The only point where the two orthogonal concepts intersect is when the object model targeted by your ORM mapping is precisely your domain model. This is possible with what EF calls "Code first" (but should really be named regular ORM), by pointing to your DDD Entities in separate EF mapping files living in a non-domain layer, and refraining from using EF artifacts such as data annotations directly in your Entity classes. This is not possible when using Database First, because the DDD "purity" part of the deal wouldn't be met.
In short, the terms collide, but they should really be conceptually considered as two different things. One is the domain object itself and the other is a pointer that can indicate the same bunch of code, but it could point to pretty much anything else.