Entity Framework Core Include(…Where)

According to the docs we have the possibility to use Where() inside Include in EF Core 5. So this code is working well:

var groups = dbContext.DocumentGroups
                      .Include(e => e.Types.Where(x => true))
                      .OrderBy(e => e.Name);

Now I have to refactor my filter (in this case simplified) – so I tried an extension method like this:

public static IEnumerable<T> MyFilter<T>(this IEnumerable<T> query)
{
    // ... do a lot of filter stuff here
    return query.Where(e => true);
}

and call it like this:

var groups = dbContext.DocumentGroups
                      .Include(e => e.Types.MyFilter())
                      .OrderBy(e => e.Name);

I get an exception:

The expression ‘e.Types.MyFilter(__dbContext_1)’ is invalid inside an ‘Include’ operation, since it does not represent a property access: ‘t => t.MyProperty’. To target navigations declared on derived types, use casting (‘t => ((Derived)t).MyProperty’) or the ‘as’ operator (‘t => (t as Derived).MyProperty’). Collection navigation access can be filtered by composing Where, OrderBy(Descending), ThenBy(Descending), Skip or Take operations. For more information on including related data, see http://go.microsoft.com/fwlink/?LinkID=746393.

How can I fix this?

Answer

I think the main thing here is that you need to provide an Expression to your include. If you want to refactor and have it as a testable, reusable independent unit you can create for example a static Expression property that you then can pass to the Include method.

Something like;

public static Expression<Func<MyStoreObject, IEnumerable<MySubType>>> MyIncludeExpression 
{
   get { return e => e.Types.Where(x => true); }
}

(MyStoreObject being the type of the DocumentGroups, MySubType being the type of the property Types enumerable.)

and call it with

var groups = dbContext.DocumentGroups
                .Include(MyClass.MyIncludeExpression)
                .OrderBy(e => e.Name);

See the documentation of the EntityFrameworkQueryableExtensions.Include Method.