Using IQueryable<T> with LINQ join tables - asp.net-mvc

Question

I have a method of type IQueryable<T> and here is it's implementation:
public IQueryable<T> Get()
{
return db.Set<T>();
}
I must write LINQ query where I want to join two tables (left join). It is Identtiy table Users and my custom table PersonalInformation which extend User registration fields and now I want to call this method in my LINQ query and it is good. Here is my Linq:
IRepository<PersonalInformation> personRepository;
IRepository<ApplicationUser> usersRepository;
var query = personRepository.Get();
var test = usersRepository.Get()
.GroupJoin(query,
n => n.Id,
m => m.UserId,
(n, ms) => new { n, ms = ms.DefaultIfEmpty() })
.SelectMany(z => z.ms.Select(m => new PersonalInfoModel
{
Name = m.Name,
LastName = m.LastName,
Email = z.n.Email
}));
But I have an error in
var test = usersRepository.Get() - System.NotSupportedException. So method Get from personRepository called good but usersRepository method return null. Where I did the mistake?? Thanks

Answer 1

It looks likely that you are having an error combining queries from two different database contexts. Your custom PersonalInformation is probably in a custom DBContext while Users is in the IdentityDBContext. See this related question. You can either:
Move all of your tables into the same context.
Avoids future confusion between these tables
More efficient if you end up with lots of associations across contexts.
A more involved solution just to get this one example working.
Query your tables separately and combine in memory.
Less scalable if you have a huge number of users.
These operators will cause EF to return the results so you can process in memory.
var people = personRepository.Get().ToList();
var users = usersRepository.Get().ToList();
var infoModels = users.GroupJoin(people,
u => u.Id,
p => p.UserId,
(u, mp) => new { n, ms = ms.DefaultIfEmpty() })
.SelectMany(z => z.ms.Select(m => new PersonalInfoModel
{
Name = m.Name,
LastName = m.LastName,
Email = z.n.Email
}));