EF Table Per Type with Navigation property on Sub Type Table

问题

I'm hoping someone out in the SO community will be able to help me out here.

Simplified Background: I'm using Entity Framework V1 to build my class structure that is outlined below, I'm using Table Per Type to persist my inherited objects:

Employee 
CaseA : Case
CaseB : Case
CaseC : Case

CaseB has a Navigational Property to Employee

I have a Repository that returns an ObjectQuery. If the type of Case is actually CaseB, I need to include the Employee object within the graph. I can't .Include("Employee") because it's not a navigational property of Case, and Employee doesn't have a .Load() method on it.

Ideally I want to be able to do this in one query, however as a fall back I'm happy that I make a call, check the Object and perform another call, something like this: (although as I stated earlier, load doesn't exist on the employee navigational property)

    //Get the case from the 
    Case myCase = new Repo<Case, Entities>.FirstOrDefault();

    if(myCase is CaseB)
       ((CaseB)myCase).Employees.load();

Am I missing something really simple here?

回答1:

Try this:

var employee = ctx.Cases
                  .OfType<CaseB>()
                  .Include("Employees")
                  .Select(x => x.Employees)
                  .FirstOrDefault();

OfType<T>() is one of the most important methods in EF when it comes to inheritance - you should familiarize yourself with it.

Essentially is filters the items in the query to be of a particular type - very similar to the conditional check your doing in your answer.

It's an IQueryable method (LINQ-Objects), but in LINQ-Entities (ObjectQuery<T>), it get's implemented as an INNER JOIN.

The above should work - just make sure you do the eager load after you do the OfType.

HTH.

回答2:

As always, after posting the question I found this and this which has pointed me towards using projection (solution below), but I was hoping to avoid that, so I'm going to leave the question open to see if there is a more elegant solution.

var Employee = caseObjectQuery.Select(x => new{
                Employee = x is CaseB ? (x as CaseB).Employee : null
            }
            ).FirstOrDefault();

Just by selecting the Object into memory, EF magically maps the related entities.

来源：https://stackoverflow.com/questions/4520498/ef-table-per-type-with-navigation-property-on-sub-type-table

标签

entity-framework

inheritance

table-per-type