@property<EOL><INDENT>def changes(self):<DEDENT>
return {key: (self._previous[key], self._instance[key])<EOL>for key in self._previous}<EOL>
Returns a dict containing just the fields which have changed on this
Document since it was created or last saved, together with both their
previous and current values

    doc['name']             # => 'bob'
    doc['name'] = 'clive'
    doc.changes             # => {'name': ('bob', clive')}