Django 1.11 uses the old manager inheritance system, unless it's
overwritten with manager_inheritance_from_future. With a class layout
like:
PolymorphicModel (abstract)
PolymorphicMPTTModel (abstract)
GenericCustomer (concrete, has objects = ...)
CustomerGroupBase (abstract, has objects = ...)
Partner (concrete, no manager)
BranchPartner (concrete, no manager)
The last level gets a normal Django Manager instead of the polymorphic
manager. Because the PolymorphicModel had a base_objects manager, this
was typically used as _default_manager. Now that the default manager is
no longer affected, it's also easier to detect why the "objects" doesn't
get the proper manager type. Using "manager_inheritance_from_future" is
recommended instead to have both the right behavior and forward
Django 2.x compatibility.
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