django orm 源码(其二)

三　QuerySet操作数据库
    1, 根据上一部分　我们来到了　QuerySet　定义了操作数据库方法
    2, UserApiKey.objects.iterator() 会调用　self._iterable_class = ModelIterable

    # 来自于 query.py 之中的　QuerySet
    def _iterator(self, use_chunked_fetch, chunk_size):
        yield from self._iterable_class(self, chunked_fetch=use_chunked_fetch, chunk_size=chunk_size)

    def iterator(self, chunk_size=2000):
        """
        An iterator over the results from applying this QuerySet to the
        database.
        """
        if chunk_size <= 0:
            raise ValueError('Chunk size must be strictly positive.')
        use_chunked_fetch = not connections[self.db].settings_dict.get('DISABLE_SERVER_SIDE_CURSORS')
        return self._iterator(use_chunked_fetch, chunk_size)

　　3,ModelIterable 定从哪个数据库中获取数据的入口 以及查询数据库

class ModelIterable(BaseIterable):
    """Iterable that yields a model instance for each row."""

    def __iter__(self):
        queryset = self.queryset
        db = queryset.db
        # 连接数据库的操作
        compiler = queryset.query.get_compiler(using=db)
        # Execute the query. This will also fill compiler.select, klass_info,
        # and annotations.
        # 执行数据库查询语句的操作
        results = compiler.execute_sql(chunked_fetch=self.chunked_fetch, chunk_size=self.chunk_size)
        select, klass_info, annotation_col_map = (compiler.select, compiler.klass_info,
                                                  compiler.annotation_col_map)
        model_cls = klass_info['model']
        select_fields = klass_info['select_fields']
        model_fields_start, model_fields_end = select_fields[0], select_fields[-1] + 1
        init_list = [f[0].target.attname
                     for f in select[model_fields_start:model_fields_end]]
        related_populators = get_related_populators(klass_info, select, db)
        for row in compiler.results_iter(results):
            obj = model_cls.from_db(db, init_list, row[model_fields_start:model_fields_end])
            for rel_populator in related_populators:
                rel_populator.populate(row, obj)
            if annotation_col_map:
                for attr_name, col_pos in annotation_col_map.items():
                    setattr(obj, attr_name, row[col_pos])

            # Add the known related objects to the model, if there are any
            if queryset._known_related_objects:
                for field, rel_objs in queryset._known_related_objects.items():
                    # Avoid overwriting objects loaded e.g. by select_related
                    if field.is_cached(obj):
                        continue
                    pk = getattr(obj, field.get_attname())
                    try:
                        rel_obj = rel_objs[pk]
                    except KeyError:
                        pass  # may happen in qs1 | qs2 scenarios
                    else:
                        setattr(obj, field.name, rel_obj)

            yield obj