EF Core GroupBy query throws: could not be translated - entity-framework

Question

Can anyone point me at the faux pas in this EF Core query?
var employers = await iqDbContext.Payrolls
.GroupBy(p => p.PayeScheme.Employer, p => p)
.Select(group => new EmployerViewModel
{
Id = group.Key.Id,
Name = group.Key.Name
})
.ToListAsync();
throws:
System.InvalidOperationException: The LINQ expression 'DbSet<Payroll>()
.Join(
inner: DbSet<PayeScheme>(),
outerKeySelector: p => EF.Property<Nullable<int>>(p, "PAYR_fk1_PAYE_SCHEME"),
innerKeySelector: p0 => EF.Property<Nullable<int>>(p0, "Id"),
resultSelector: (o, i) => new TransparentIdentifier<Payroll, PayeScheme>(
Outer = o,
Inner = i
))
.Join(
inner: DbSet<Employer>(),
outerKeySelector: p => EF.Property<Nullable<int>>(p.Inner, "PychFk1Employer"),
innerKeySelector: e => EF.Property<Nullable<int>>(e, "Id"),
resultSelector: (o, i) => new TransparentIdentifier<TransparentIdentifier<Payroll, PayeScheme>, Employer>(
Outer = o,
Inner = i
))
.Where(p => p.Inner != null && __accessiblePayrollIds_0.Contains(p.Outer.Outer.Id))
.GroupBy(
keySelector: p => p.Inner,
elementSelector: p => p.Outer.Outer)' could not be translated. Either rewrite the query in a form that can be translated, or switch to client evaluation explicitly by inserting a call to 'AsEnumerable', 'AsAsyncEnumerable', 'ToList', or 'ToListAsync'. See https://go.microsoft.com/fwlink/?

Answer 1

If you need correct grouping, you have to specify which columns to group, not whole object.
var employers = await iqDbContext.Payrolls
.GroupBy(p => new { p.PayeScheme.Employer.Id, p.PayeScheme.Employer.Name })
.Select(group => new EmployerViewModel
{
Id = group.Key.Id,
Name = group.Key.Name,
})
.ToListAsync();