Code review comment for lp:~thisfred/u1db/iterate-over-list-of-dicts

 + subfields = self.field.split('.')

I would move this to the constructor

255 + return extract_field(raw_doc, subfields)

+def extract_field(raw_doc, subfields):
206 + if not isinstance(raw_doc, dict):
207 + return []
208 + val = raw_doc.get(subfields.pop(0))

I would use a index argument and avoid the pop (especially pop(0)) and copy

216 + subfields = []
this line is not needed

same for these:
223 + if val is None:
224 + return []

line 209-210 cover that