Question

What’s wrong with this Lambda query? I want to be able to only include ProjectDocs of a certain type, there could be many types of ProjectDocs

 ICollection<Project> projects = db.Projects
      .Include(i => i.ProjectDoc.OfType<Cover>().Where(s => s.Status == "Active"))
      .Include(i => i.ProjectDoc.OfType<Summary>().Where(s => s.Status == "Active"))
      .Include(i => i.User)
      .Include(i => i.ProjectTag.Select(t => t.Tag)).ToList();

I have a model ProjectDoc with the derived classes Cover, Segment and Summary. Should I just include ProjectDoc and use the discriminator column in a condition later? Some of the types could have a large number of results, others just a few.

The Error I get…

 The Include path expression must refer to a navigation property defined 
 on the type. Use dotted paths for reference navigation properties and the 
 Select operator for collection navigation properties.
 Parameter name: path

The Navigation Property on “Project” is ProjectDoc. There is no navigation property for the derived classes. When I tried that I got tons of extra keys.

Answer 1

This scenario is not supported – you can only load or not load a set of related entities but you can not apply filter expressions to load only a subset of the entities.

The API documentation for Include() lists the different expressions that are supported and states that the method just delegates the work to an underlying Include() method taking a string as argument, for example ObjectQuery.Include(). The documentation of this method and the linked page Shaping Query Results make it more or less obvious that this is not supported.