SQL Server查询创建不同的返回类型

Question

I'll go straight with the code, then explain my problem, this way it will be easier. Just to give a heads up, I use entity framework 7 with SQL Server 2014, ASP.NET 5 and MVC 6.

So, I've following classes :

public class Base {
    public int BaseId { get; set; }
    public string Name { get; set; }
}
public class DerivedOne : Base { 
    public virtual List<OtherDBObject> ListOne { get; set; }  
}
public class DerivedTwo : Base {
    public string SomeContent { get; set; }
}

And I created repository that retrievs Base objects from database. Problem is, DerivedOne and DerivedTwo have different members - how can I know which type to return (which members to query further)?

public class Repo : IRepo {
    public async Task<Base> GetById(int id) {
        // I though that this method should work something like this (its pseudo code) :
        context.Base.Where(b => b.BaseId == id)
            .switch(obj.type) {
                case DerivedOne : query for OtherDBObjects
                case DerivedTwo : query for string content somewhere else
                ...
            }).First();
    }
}

My goal is to have very abstract Base class containing just a few basic fields and derived ones can be completely diffrent.

PS If you have different structure for this task, feel free to post it!

Answer 1

You can write good stuff by using generics and extensions methods, something like the following:

public static Task<TEntity> GetByIdAsync<TEntity>( 
    this DbSet<TEntity> set, int id)  where TEntity : Base
{
    return set.FirstOrDefaultAsync(e => e.Id == id);
}

Sample usage:

DerivedOne entity = await DbContext.DerivedOnes.GetByIdAsync(id);