实体或复杂类型''不能在LINQ to Entities查询，三表外部联接中构造

Question

I see some related questions on stack, but not sure how to still solve it for my problem as I'm doing a three table left outer join. RoleUser and Role are an entity object

var query = from u in Context.Users
            join ru in Context.RoleUsers on u.Id equals ru.UserId into plv
            from x in plv.DefaultIfEmpty(new RoleUser())
            join r in Context.Roles on x.RoleId equals r.Id into pii
            from y in pii.DefaultIfEmpty(new Role())
            orderby u.Id, y.Id
            select new UserWithRole() { Id = u.Id, User = u, Role = y };

Answer 1

As the error explains you cannot create in a linq query they mapped types ( RoleUser and Role ). Instead use the DefaultIfEmpty() . Something along the following:

var roles = from ru in Context.RoleUsers 
            join r in Context.Roles on ru.RoleId equals r.Id
            select new { ru.UserId, role = r };

var query = from u in Context.Users
            join r in roles on u.Id equals r.UserId into plv
            from x in plv.DefaultIfEmpty()
            select new UserWithRole() { Id = u.Id, User = u, Role = x.role };

However I'd suggest look into navigation properties - that way you will not have to construct the joins and work with it as if it is "normal" objects.

Answer 2

The problem is DefaultIfEmpty(new RoleUser()) and DefaultIfEmpty(new Role()) calls, because EF cannot translate creation of a constant entity object to SQL.

So you need to use the parameterless overload of DefaultIfEmpty() and deal with null objects in the materialized query result.

If you don't need a IQueryable<T> result, you can use intermediate anonymous type projection, which to LINQ to Objects context by using AsEnumerable() and do the final desired projection:

var query = (from u in Context.Users
             join ru in Context.RoleUsers on u.Id equals ru.UserId into userRoles
             from ru in userRoles.DefaultIfEmpty()
             join r in Context.Roles on ru.RoleId equals r.Id into roles
             from r in roles.DefaultIfEmpty()
             orderby u.Id, r.Id
             select new { User = u, Role = r }
            )
            .AsEnumerable() // db query ends here
            .Select(x => new UserWithRole
            {
                Id = x.User.Id,
                User = x.User,
                Role = x.Role ?? new Role() // now this is supported
            });