如何根据涉及继承的模型更改来修改此联接？

Question

I made a change to the model of my project, and there is a join that I can't seem to figure out how to change in response to the model change. Here is the original model setup, and the join code:

           query = query.Join(db.Reports,
                x => new
                {
                    x.SourceUser,
                    x.TargetUser
                },
                x2 => new
                {
                    SourceUser = x2.TargetUser,
                    TargetUser = x2.SourceUser
                },
                (x, x2) => new { x, x2 }).Where(f => (f.x.SourceUser == user)).Select(p => p.x);

The result of this join is that the resulting query provides only the report relationships that are mutual (as in, both parties have reported eachother). This join works perfectly fine, until I make the model change.

Here is the new model setup

and here is my attempt at modifying the join code

              query = query.Join(db.Reports,
                x => new
                {
                    x.SourceUser,
                    x.TargetReportable
                },
                x2 => new
                {
                    SourceUser = x2.TargetReportable,
                    TargetReportable = x2.SourceUser
                },
                (x, x2) => new { x, x2 }).Where(f => (f.x.SourceUser == user)).Select(p => p.x);

This results in the following error:

The type arguments for method 'System.Linq.Enumerable.Join(System.Collections.Generic.IEnumerable, System.Collections.Generic.IEnumerable, System.Func, System.Func, System.Func) cannot be inferred from the usage. Try specifying the type arguments explicitly.

Answer 1

I'm assuming that SourceUser is of type UserAccount and TargetReportable is of type Reportable. The lambda is comparing apples to oranges,. you need to cast sourceuser to type Reportable since it inherits Reportable.

 query = query.Join(db.Reports,
                x => new
                {
                    T = (Reportable)x.SourceUser,
                    x.TargetReportable
                },
                x2 => new
                {
                    SourceUser = x2.TargetReportable,
                    TargetReportable = (Reportable)x2.SourceUser
                },
                (x, x2) => new { x, x2 }).Where(f => (f.x.SourceUser == user)).Select(p => p.x);