OData service on legacy database

Hi,

What is the best option to build an OData service on a legacy database? 

All the tutorials are creating a database from scratch to match the entity framework model. 

How does it work with an existing database that does not match the service model being exposed?

For example, I have these 2 db tables:

Science_Data { country_id varchar(3), year int, fig decimal }

Arts_Data { country_id varchar(3), year int, fig decimal }

and I want to expose it using this path /Category('Science')/Data and /Category('Arts')/Data (The ids are just for the example).

I was able to achieve some success by creating a service model that contains Category and Data entities and creating a controller:

public class CategoryController : ODataController
{
        [EnableQuery]
        [ODataRoute("/Category('{id}')/Data")]
        public IQueryable<Sector> Get([FromODataUri]string id)
        {
            if (id == "Science")
            { 
                return db.Science_Data.Select(i => new Data() { CountryId = i.country_id, Year = year, Figure = fig });
            }
            else if(id == "Arts")
            {
                return db.Args_Data.Select(i => new Data() { CountryId = i.country_id, Year = year, Figure = fig });
            }
        }
}

This works to a certain extents and supports query options and the filtering will happen at the database level. 

But I'm having problems supporting $expand options so I'm wondering if my approach is correct to begin with.

Thanks