None can not be compared to a class so we always use strings to order before
grouping. Also dictionary can not be compared neither.
issue8265
review253351002
Field may have a context defined so when it instantiate a relational record,
the field should set this context like when the record is read from the
database in ModelStorage.__getattr__.
And when such field is read as related field, the context should also be set.
issue8225
review287171002
Cédric Krier