File field for files stored in S3 that provides easy access to previous versions

s3_file_field.py

'''
Adds access to previous versions to a model's file field.

Requires django-storages, and you must either be using S3Boto3Storage 
by default, or you must specify it as the storage class in the field
definition.

class SampleModel(models.Model):
    file = S3FileField(upload_to='path/to/dir')

>> record = SampleModel.objects.all()[0]
>> record.file
<S3FieldFile: path/to/dir/filename.docx>
>> record.file.versions
<S3FileVersions: {1: <Key: bucket_name,path/to/dir/filename.docx>, 2: <Key: bucket_name,path/to/dir/filename.docx>, 
3: <Key: bucket_name,path/to/dir/filename.docx>}>
>> record.file.versions.first
<Key: bucket_name,path/to/dir/filename.docx>
>> record.file.versions.first.version
1
>> record.file.versions.first.date
datetime.datetime(2019, 1, 1, 8, 25, 45)
>> record.file.versions.first.url
https://s3.amazonaws.com:443/bucket_name/path/to/dir/filename.docx?Signature=...&Expires=...&AWSAccessKeyId=...&versionId=...
>> record.file.versions[2].date
datetime.datetime(2019, 1, 3, 11, 8, 11)
>> record.file.versions[2].version
2
>> record.file.versions.previous.version
2
>> record.file.versions.current.version
3
'''

class S3FileVersions:
    def __init__(self, current, versions, expires_in=None):
        expires_in = expires_in or 100
        self.versions = {}
        for vv_idx, vv in enumerate(sorted(versions, key=lambda x: x.last_modified)):
            setattr(vv, 'date', datetime.datetime.strptime(vv.last_modified, '%Y-%m-%dT%H:%M:%S.%fZ'))
            setattr(vv, 'version', vv_idx + 1)
            setattr(vv, 'url', vv.generate_url(expires_in))
            self.versions[vv_idx + 1] = vv
            if vv.version_id == current.version_id:
                current = vv
        self.current = current

    def __iter__(self):
        return iter(self.versions.values())

    @property
    def previous(self):
        return self.versions[max(
            kk for kk, vv in 
            self.versions.items()
            if vv.version_id != self.current.version_id
        )]

    @property
    def first(self):
        return self.versions[min(self.versions.keys())]

    def __getitem__(self, version):
        return self.versions[version]

    def __repr__(self):
        return '<S3FileVersions: {}>'.format(repr(self.versions))


class S3FieldFile(FieldFile):
    @property
    def versions(self):
        current = self.storage.bucket.get_key(self.name)
        return S3FileVersions(current, self.storage.bucket.list_versions(self.name))
        

class S3FileField(models.FileField):
    attr_class = S3FieldFile