code: def _find_field(cls, field, doc):
          
          path = field.split(".")
          try:
              for key in path:
                  doc = doc[key]
              return [(path, doc)]
          except (KeyError, TypeError):
              return []
docs: Find the field in the document which matches the given field.
  
          The field may be in dot notation, eg "a.b.c". Returns a list with
          a single tuple (path, field_value) or the empty list if the field
          is not present.