問題來自您的list(reviews)
表達式 - 這迫使RawQuerySet試圖以最簡單的方式從最初的查詢中構建完整的模型實例(可能不是最有效的方式,但這是另一個故事)。如果沒有這行,假設你沒有訪問相關的對象,就只有一個查詢做,因爲你可以在這個片段看到:
>>> from survey.models import Question
>>> from django.db import connection
>>> import pprint
>>> connection.queries
[]
>>> raw = Question.objects.raw("select * from survey_question")
>>> for q in raw:
... print q.id
...
1
2
3
4
>>> connection.queries
[{'time': '0.000', 'sql': 'select * from survey_question'}]
>>> list(raw)
[<Question: Survey Essai1 (root) question #1 - Depuis combien de temps programmez vous ?>, <Question: Survey Essai1 (root) question #2 - Comment avez vous débuté ?>, <Question: Survey Essai1 (root) question #3 - Quel est votre niveau de formation>, <Question: Survey Essai1 (root) question #4 - Cette formation porte-t-elle sur l'informatique ?>]
>>> pprint.pprint(connection.queries)
[{'sql': 'select * from survey_question', 'time': '0.000'},
{'sql': 'select * from survey_question', 'time': '0.000'},
{'sql': 'SELECT `survey_survey`.`id`, `survey_survey`.`user_id`, `survey_survey`.`title`, `survey_survey`.`notes`, `survey_survey`.`description`, `survey_survey`.`instructions`, `survey_survey`.`date_created`, `survey_survey`.`starts_on`, `survey_survey`.`ends_on` FROM `survey_survey` WHERE `survey_survey`.`id` = 1 ',
'time': '0.001'},
{'sql': 'SELECT `auth_user`.`id`, `auth_user`.`username`, `auth_user`.`first_name`, `auth_user`.`last_name`, `auth_user`.`email`, `auth_user`.`password`, `auth_user`.`is_staff`, `auth_user`.`is_active`, `auth_user`.`is_superuser`, `auth_user`.`last_login`, `auth_user`.`date_joined` FROM `auth_user` WHERE `auth_user`.`id` = 1 ',
'time': '0.001'},
{'sql': 'SELECT `survey_survey`.`id`, `survey_survey`.`user_id`, `survey_survey`.`title`, `survey_survey`.`notes`, `survey_survey`.`description`, `survey_survey`.`instructions`, `survey_survey`.`date_created`, `survey_survey`.`starts_on`, `survey_survey`.`ends_on` FROM `survey_survey` WHERE `survey_survey`.`id` = 1 ',
'time': '0.000'},
{'sql': 'SELECT `auth_user`.`id`, `auth_user`.`username`, `auth_user`.`first_name`, `auth_user`.`last_name`, `auth_user`.`email`, `auth_user`.`password`, `auth_user`.`is_staff`, `auth_user`.`is_active`, `auth_user`.`is_superuser`, `auth_user`.`last_login`, `auth_user`.`date_joined` FROM `auth_user` WHERE `auth_user`.`id` = 1 ',
'time': '0.000'},
{'sql': 'SELECT `survey_survey`.`id`, `survey_survey`.`user_id`, `survey_survey`.`title`, `survey_survey`.`notes`, `survey_survey`.`description`, `survey_survey`.`instructions`, `survey_survey`.`date_created`, `survey_survey`.`starts_on`, `survey_survey`.`ends_on` FROM `survey_survey` WHERE `survey_survey`.`id` = 1 ',
'time': '0.000'},
{'sql': 'SELECT `auth_user`.`id`, `auth_user`.`username`, `auth_user`.`first_name`, `auth_user`.`last_name`, `auth_user`.`email`, `auth_user`.`password`, `auth_user`.`is_staff`, `auth_user`.`is_active`, `auth_user`.`is_superuser`, `auth_user`.`last_login`, `auth_user`.`date_joined` FROM `auth_user` WHERE `auth_user`.`id` = 1 ',
'time': '0.000'},
{'sql': 'SELECT `survey_survey`.`id`, `survey_survey`.`user_id`, `survey_survey`.`title`, `survey_survey`.`notes`, `survey_survey`.`description`, `survey_survey`.`instructions`, `survey_survey`.`date_created`, `survey_survey`.`starts_on`, `survey_survey`.`ends_on` FROM `survey_survey` WHERE `survey_survey`.`id` = 1 ',
'time': '0.000'},
{'sql': 'SELECT `auth_user`.`id`, `auth_user`.`username`, `auth_user`.`first_name`, `auth_user`.`last_name`, `auth_user`.`email`, `auth_user`.`password`, `auth_user`.`is_staff`, `auth_user`.`is_active`, `auth_user`.`is_superuser`, `auth_user`.`last_login`, `auth_user`.`date_joined` FROM `auth_user` WHERE `auth_user`.`id` = 1 ',
'time': '0.000'}]
>>>
現在的問題是:爲什麼要使用一個RawQueryset在所有的上面的計算?
我假設第一個avg_rating = avg_rating/float(count)
行在循環中的一個 - 是一個複製粘貼錯誤(如果不是,我會很想知道更多關於您的「平均」的定義)。然後,您只需要一個查詢(不需要循環,也不需要python計算)來要求數據庫執行計算。你可以做到這一點無論使用原始的SQL:
select avg(stars) from reviews;
或使用ORM的聚合功能(CF https://docs.djangoproject.com/en/1.4/topics/db/aggregation/):
>>> from django.db.models import Avg
>>> Book.objects.all().aggregate(Avg('price'))