'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.'

@classmethod
def _find_field(cls, field, doc):

    path = field.split('.')
    try:
        for key in path:
            doc = doc[key]
        return [(path, doc)]
    except (KeyError, TypeError):
        return []
