'Sets up the select_related data structure so that we only select
certain related models (as opposed to all models, when
self.select_related=True).'

def add_select_related(self, fields):

    field_dict = {}
    for field in fields:
        d = field_dict
        for part in field.split(LOOKUP_SEP):
            d = d.setdefault(part, {})
    self.select_related = field_dict
    self.related_select_cols = []
    self.related_select_fields = []
