Preventing DeferredAttribute.__ get__() unnecessary calls.

[Sultan]

To retrieve a deferred model attributes, the get method is called twice. This is because it uses the getattr() function, which in turn causes the get method to be called again.

    def __get__(self, instance, cls=None):
        """
        Retrieve and caches the value from the datastore on the first lookup.
        Return the cached value.
        """
        if instance is None:
            return self
        data = instance.__dict__
        field_name = self.field.attname
        if field_name not in data:
            # Let's see if the field is part of the parent chain. If so we
            # might be able to reuse the already loaded value. Refs #18343.
            val = self._check_parent_chain(instance)
            if val is None:
                instance.refresh_from_db(fields=[field_name])
                **val = getattr(instance, field_name)**
            data[field_name] = val
        return data[field_name]

To prevent this unnecessary call, we can simply extract the value from the instance dict (at that moment it already contains the reloaded value):

    def __get__(self, instance, cls=None):
        """
        Retrieve and caches the value from the datastore on the first lookup.
        Return the cached value.
        """
        if instance is None:
            return self
        data = instance.__dict__
        field_name = self.field.attname
        if field_name not in data:
            # Let's see if the field is part of the parent chain. If so we
            # might be able to reuse the already loaded value. Refs #18343.
            val = self._check_parent_chain(instance)
            if val is None:
                instance.refresh_from_db(fields=[field_name])
                # Now the instance dict contains the reloaded data.
                # Using getattr() will do extra call to __get__ method.
                **val = data[field_name]**
            data[field_name] = val
        return data[field_name]

This reduces the number of method calls.

[Mariusz Felisiak]

It looks that val = getattr(instance, field_name) is redundant.

[Sultan]

Replying to felixxm: Yes you are right. I changed the code. Now it has become much easier and better. Thank you.

    def __get__(self, instance, cls=None):
        """
        Retrieve and caches the value from the datastore on the first lookup.
        Return the cached value.
        """
        if instance is None:
            return self
        data = instance.__dict__
        field_name = self.field.attname
        if field_name not in data:
            # Let's see if the field is part of the parent chain. If so we
            # might be able to reuse the already loaded value. Refs #18343.
            val = self._check_parent_chain(instance)
            if val is None:
                instance.refresh_from_db(fields=[field_name])
            else:
                data[field_name] = val
        return data[field_name]

[Mariusz Felisiak <felisiak.mariusz@…>]

In 678c8df:

Fixed #31680 -- Removed unnecessary getattr() call in DeferredAttribute.get().

refresh_from_db() loads fields values.