Help with Lambda query syntax

I am trying to validate a unique combination of data in a table. I am using
devart Entity Framework.

I am trying to setup the validation on entity with Lightswitch as the application.

The Table I am checking for values already existing is:  StuntDetails.
It has a foreign key for packageID and StuntId.   I want to check that the
combination of PackageID and StuntID do not already exist in StuntDetails when
inserting.  If it does, I want to raise an error.

In Entity Framework, it makes the foreign key and object linked to the parent table and for
referencing my PackageID I have to enter the entity  Package.PackageId.

Can you please help me figure this out so it works correctly?

Here is what I have coded but I don't have the lambda code correct:

            if (this.StuntDetails.Where(x => x.Package.PackageId == entity.Package.PackageId).FirstOrDefault() && x.Stunt.StuntId == entity.Stunt.StuntId).FirstOrDefault())
            {
                results.AddPropertyError(
                    "This PackageId has already been entered for this Stunt. PackageId and StuntId must be unique for Stunt Detail.",
                    entity.Details.Properties.StuntDetailId);
            }

Thank you!