This ensures that model.save works when a field has the many flag set,
but the object has no attribute of that name, and the caller appends
to the attribute list, like this:

      user.key_credential_link.append(link)

When we get to save, and are doing this:

      value = getattr(self, attr)
      old_value = getattr(existing_obj, attr)
      if value != old_value:
         # commit the change

the .append() will have added the item to both value and old_value
because they are the same list. But not any more.

This was a problem because the Field instance is attached to the
model class, not the model instance.

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Gary Lockyer <gary@catalyst.net.nz>

This will soon be needed by samba-tool, and is also going to be used
in some tests.

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Gary Lockyer <gary@catalyst.net.nz>

This argument is actually an instance of SamDB (which inherits from Ldb).

This should have been called samdb.

Signed-off-by: Rob van der Linde <rob@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>

Signed-off-by: Rob van der Linde <rob@catalyst.net.nz>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>