使用实体框架转换为IQueryable时为空值

Question

I'm working with EF5 and I have a generic repository with a GetById method that can receive the include properties wanted like this one:

public virtual T GetByIdIncluding(long id, params Expression<Func<T, object>>[] includeProperties)
{
    IQueryable  <T> query = DbContext.Set<T>().Find(id) as IQueryable<T>;

    foreach (var includeProperty in includeProperties)
    {
        query = query.Include(includeProperty);
    }

    return query.FirstOrDefault();
}

DbContext.Set<T>().Find(id) return the correct object, but when I cast it with as IQueryable then the value of variable query becomes null.

Why? How can I select only the entity with the Id sent by parameter and all the includings sent by parameter?

Example of usage:

var houses = Uow.Types.GetByIdIncluding(id, tt => tt.Houses);

Thanks in advance! Guillermo

Answer 1

DbContext.Set<T>().Find(id) returns single entity.

IQueryable<T> is a DbContext.Set<T>() .

---

You can't include some properties after you selected single entity by id. Also you can't use DbSet<T>.Find after you included some properties, because result will be IQueryable<T> . What you can do - include properties and filter result by id later:

public static IQueryable<T> Including(
                          params Expression<Func<T, object>>[] includeProperties)
{            
    IQueryable<T> query = DbContext.Set<T>();

    foreach (var includeProperty in includeProperties)    
        query = query.Include(includeProperty);    

    return query;
}

And later:

var types = Uow.Types.Including(tt => tt.Houses)
                     .SingleOrDefault(tt => tt.Id == id);

Also you can create method GetByIdIncluding in your concrete repository classes (not in generic repository):

public static Foo GetByIdIncluding(long id, 
                 params Expression<Func<Foo, object>>[] includeProperties)
{
    return Including(includeProperties).SingleOrDefault(f => f.Id == id);
}

BTW consider using lazy-loading (it's enabled by default).

Answer 2

http://msdn.microsoft.com/en-us/library/gg696418%28v=vs.103%29.aspx DbSet.Find method returns an entity, not an IQueryable of your entity. Try setting the Include on the Set before calling the Find method.

Answer 3

You need to move the Find to after the loop:

public virtual T GetByIdIncluding(long id, params Expression<Func<T, object>>[] includeProperties)
{
    var query = DbContext.Set<T>();

     foreach (var includeProperty in includeProperties)
     {
         query = query.Include(includeProperty);
     }

    return query.Find(id);
}