Entity Framework Design - Multiple "Views" for the data

Question

I have a design question related to Entity Framework entities.

I have created the following entity:

public class SomeEntity {
    // full review details here
}

This entity has as an example 30 columns. When I need to create a new entity this works great. I have all of the required fields in order to insert into the database.

I have a few places in my app where I need to display some tabular data with some of the fields from SomeEntity, but I don't need all 30 columns, maybe only 2 or 3 columns.

Do I create an entirely new entity that has only the fields I need (which maps to the same table as SomeEntity, but only retrieves the column I want?)

Or does it make more sense to create a domain class (like PartialEntity) and write a query like this:

var partialObjects = from e in db.SomeEntities
                     select new PartialEntity { Column1 = e.Column1, Column2 = e.Column2 };

I am not sure what the appropriate way to do this type of thing. Is it a bad idea to have two entities that map to the same table/columns? I would never actually need the ability to create a PartialEntity and save it to the database, because it wouldn't have all of the fields that are required.

Answer 1

Your first approach is not possible. EF doesn't support multiple entities mapped to the same table (except some special cases like TPH inheritance or table splitting).

The second case is common scenario. You will create view model for your UI and either project your entity to view model directly in query (it will pass from DB only columns you project) or you will query whole entity and make conversion to view model in your application code (for example by AutoMapper as @Fernando mentioned).

If you are using EDMX file for mapping (I guess you don't because you mentioned ef-code-first) you can use third approach which takes part from both mentioned approaches. That approach defines QueryView - it is EF based view on the mapped entity which behaves as a new read only entity. Generally it is reusable projection stored directly in mapping.