Wouldn't it be simpler to do something like this:
```python
exists = models.InstanceExists.all()
for exist in exists:
    image_metadata = models.RawDataImageMeta.objects.filter(raw_id=exists.raw.id)[0]
    exists.os_architecture = image_metadata.os_architecture
    exists.os_distro = image_metadata.os_distro
    ...
    exists.save()
```
There is a one-to-one relation between exists RawData and InstanceExists, so there is no benefit to trying to do multiple updates at the same time.