Avoid validating foreign key checks if the key never changed

This leverages our custom full_clean implementation (#141) in
combination with the LoadedValues mixin.  This allows us to know when
a value did not change and avoid validating the foreign key just
before save, because the foreign key was fine as it was stored, so it
cannot suddenly become invalid.

For complex models with many relations or which are saved a lot, this
makes a big difference, as the reduced number of queries can make
things quite a bit more efficient.

binder/models.py

@@ -476,7 +476,7 @@ def save(self, *args, **kwargs):
 		return super().save(*args, **kwargs)
 
 
-	def full_clean(self, *args, **kwargs):
+	def full_clean(self, exclude=None, *args, **kwargs):
 		# Determine if the field needs an extra nullability check.
 		# Expects the field object (not the field name)
 		def field_needs_nullability_check(field):
@@ -487,10 +487,19 @@ def field_needs_nullability_check(field):
 			return False
 
 
+		# Gather unchanged fields if LoadedValues mixin available, to
+		# avoid querying uniqueness constraints for unchanged
+		# relations (an useful performance optimization).
+		if hasattr(self, 'field_changed'):
+			exclude = set(exclude) if exclude else set()
+			for f in self.binder_concrete_fields_as_dict(skip_deferred_fields=True):
+				if not self.field_changed(f):
+					exclude.add(f)
+
 		validation_errors = defaultdict(list)
 
 		try:
-			res = super().full_clean(*args, **kwargs)
+			res = super().full_clean(exclude=exclude, *args, **kwargs)
 		except ValidationError as ve:
 			if hasattr(ve, 'error_dict'):
 				for key, value in ve.error_dict.items():

binder/plugins/loaded_values.py

@@ -27,14 +27,14 @@ def field_changed(self, *fields):
             return True
 
         for field in fields:
+            field_def = self._meta.get_field(field)
+
             try:
-                current_value = getattr(self, field)
+                current_value = getattr(self, field_def.attname)
             except ObjectDoesNotExist:
                 current_value = None
 
-            if isinstance(current_value, Model):
-                current_value = current_value.pk
-            elif isinstance(current_value, FieldFile):
+            if isinstance(current_value, FieldFile):
                 current_value = current_value.name
                 if current_value is None:
                     current_value = ''