Using Entity Framework with Existing Models

Question

I am working on a MVC4 project which will need to use a number of different databases, each with a few stored procedures for searching. The site is an asset search tool which needs to query various existing systems. If I allow the EF to generate models on its own, I will end up with a Model for each procedure I use in each database.

What I would prefer is to have my own POCO model already defined and the EF maps its results to that Model. So regardless of what database the data is taken from it maps back to that same Model. The column names in each database differ slightly so it would really need to be mapping columns to model properties.

There is no writing back to the database, it purely selects data out.

On the 'Edit Function Import' form I can create a model based on the results. There is also an option to view 'Function Import Mapping' but it does not appear to do what I am looking for.

Has anyone else tried this?

Added an image to help explain the issue

The closest to this I have managed so far is to have EDMX1 query 2 databases. This only works because they are on the same Db server. I had to fully qualify the Db names in the stored procedure. I could then use 1 EF Model as a return type for the 2 queries. That Model still is not usable in another EDMX though, so if I need to connect to a different Db server, I still cannot share the Model. So the problem is not solved.

Here is image of current progress.

Answer 1

Function Import Mapping is for mapping stored procedure / function calls to EF code. It's not really relevant here, unless you're using stored procs (which is not the way to go 90% of the time with EF - only use stored procs for more complex procedures).

An EF context, by its very nature, can only have a single database associated with it. You need to create multiple contexts in order to access multiple databases at once.

What I would do in your case is create a database-first schema (.edmx) file for each database, then write a service layer abstraction above it that allows you to flatten the data into your expected model. This is the kind of thing I do all the time, regardless of how many databases I'm working on at once. You've almost outlined this in your first diagram. The service layer may have multiple classes (for example, for a blog website you might have BlogService, UserService, CommentService etc), each of which contain methods that you call from you application layer.

I've put a quick diagram together that might help to explain

http://www.gliffy.com/go/publish/image/4818386/L.png

The service layer does all of your EF work, and your application layer (or business layer, whatever you want to call it) will do all of your business logic.

This setup lends itself well to TDD and Dependency Injection / IoC. Everything is neat and nicely separated.