I've just started working with the Entity framework and I'm confused about how the classes normally in the business layer fit in with the entities that are created by the Entity Framework.
When working with classic ADO.NET, I would have a class called Customer for example and then another class called DALCustomer to handle database interaction, in this structure I would have put the code to do calculations, filtering and delcare an instance of the DAL withing Customer for saving, updating and deleting in the Customer class.
With the Entity Framework, if you have a table called Customer, the Entity framework creates an entity called Customer and this is where my confusion begins, does this entity remove the need for a Customer in the business layer? So in essence all the fields and methods that normally go in the business layer go in the entity generated by the Entity Framework? Or should a class still exist in the business layer called CustomerBL for example, that still contains the fields and methods needed to accomplish the business logic required for calculations, filtering and still needs an instance of the EF DAL declared to handle data access?
If there should be a business class, in this case CustomerBL, one other question jumps to mind, should the fields that are created in the customer entity be recreated in CustomerBL or should an instance of the Customer entity be declared in CustomerBL so there would be no need to have the fields declared in 2 locations?
Entity framework was designed with separation between data model and conceptual model in mind. It supports inheritance, entity splitting (not EF core), table splitting, complex types or owned types, and transparent many-to-many associations (not EF core), all of which allow molding the domain model to one's needs without being constrained too much by the data store model. EF core supports shadow properties which can be used to hide cross-cutting concerns from the exposed class model.
The code-first approach allows working with POCOs in which only a few properties are mapped to data store columns and others serve domain goals. Model-first and Database-first generate partial classes, allowing one to extend the generated code.
Of course this separation of conceptual model and store model can only succeed to a certain extent. Some things work against this goal of persistence ignorance. For instance -
If lazy loading is desirable, it is necessary to declare navigation properties as
virtual, so EF can override them in proxy types. Domain-driven design (DDD) would encourage using
virtual only when polymorphism is required.
It is very convenient to have primitive foreign key properties (say,
ParentId) accompanying the "real" associations (a
Parent reference). Purists consider this a violation of DDD principles.
The EF class model will is part of a data access layer and should primarily serve that goal. Therefore, it will contain many reciprocal relationships, in order to benefit from navigation properties as much as possible when writing LINQ queries. These mutual relationships are another violation of DDD principles.
There is a large number of differences between LINQ-to-objects and LINQ-to-entities. You just can't ignore the fact that you are LINQ-ing against a totally different universe than objects in memory. This is referred to as tight coupling, or leaky abstraction.
EF can only map concrete classes, no interfaces.
But then... generally I'm happy with using generated EF classes or POCOs from a code-first model as domain classes. So far, I've never seen a frictionless transition from one data store or ORM to another, if it happens at all. Persistence ignorance is a fiction. Idiosyncrasies from the DAL easily leave a footprint in the domain. Only when you have to code for different data stores/models or when stores/models are expected to change relatively often it pays off to minimize this footprint as much as possible or abstract it away completely.
Another factor that may promote EF classes as domain classes is that many applications today have multiple tiers, where (serialized) different view models or DTOs are sent to a client. Using domain classes in UIs hardly ever fits the bill. You may as well use the EF class model as the domain and have services return dedicated models and DTOs as required by a UI or service consumers. Another abstraction layer may be more of a burden than a blessing, if only performance-wise.