Add flag to prevent operations on manytomany field w/unsaved instances.

By default, attempting to read or write a many-to-many field on an
unsaved model instance will now raise an exception. To disable this
behavior, specify `prevent_unsaved=False` when initializing your
ManyToManyField.

Refs #2765

peewee.py

@@ -5574,6 +5574,9 @@ def __get__(self, instance, instance_type=None, force_query=False):
                     return [getattr(obj, self.dest_fk.name) for obj in backref]
 
             src_id = getattr(instance, self.src_fk.rel_field.name)
+            if src_id is None and self.field._prevent_unsaved:
+                raise ValueError('Cannot get many-to-many "%s" for unsaved '
+                                 'instance "%s".' % (self.field, instance))
             return (ManyToManyQuery(instance, self, self.rel_model)
                     .join(self.through_model)
                     .join(self.model)
@@ -5582,6 +5585,10 @@ def __get__(self, instance, instance_type=None, force_query=False):
         return self.field
 
     def __set__(self, instance, value):
+        src_id = getattr(instance, self.src_fk.rel_field.name)
+        if src_id is None and self.field._prevent_unsaved:
+            raise ValueError('Cannot set many-to-many "%s" for unsaved '
+                             'instance "%s".' % (self.field, instance))
         query = self.__get__(instance, force_query=True)
         query.add(value, clear_existing=True)
 
@@ -5590,7 +5597,7 @@ class ManyToManyField(MetaField):
     accessor_class = ManyToManyFieldAccessor
 
     def __init__(self, model, backref=None, through_model=None, on_delete=None,
-                 on_update=None, _is_backref=False):
+                 on_update=None, prevent_unsaved=True, _is_backref=False):
         if through_model is not None:
             if not (isinstance(through_model, DeferredThroughModel) or
                     is_model(through_model)):
@@ -5604,6 +5611,7 @@ def __init__(self, model, backref=None, through_model=None, on_delete=None,
         self._through_model = through_model
         self._on_delete = on_delete
         self._on_update = on_update
+        self._prevent_unsaved = prevent_unsaved
         self._is_backref = _is_backref
 
     def _get_descriptor(self):