Django save vs update to update the database

Question

I m writing a Django app  and I need a function to update a field in the database   Is there any reason to do one of these methods rather than the other   def save db field name field value       obj   MyModel objects get name name      obj field   value     obj save    def update db field name field value       MyModel objects get name name  update field value    It seems like the second is better because it does it in one DB call instead of two   Is there a reason why fetching  then updating is any better

User · Answer

Using update directly is more efficient and could also prevent integrity problems  From the official documentation https   docs djangoproject com en 3 0 ref models querysets  django db models query QuerySet update  If you   re just updating a record and don   t need to do anything with the model object  the most efficient approach is to call update    rather than loading the model object into memory  For example  instead of doing this  e   Entry objects get id 10  e comments on   False e save       do this  Entry objects filter id 10  update comments on False   Using update   also prevents a race condition wherein something might change in your database in the short period of time between loading the object and calling save

User · Answer

Update will give you better performance with a queryset of more than one object  as it will make one database call per queryset    However save is useful  as it is easy to override the save method in your model and add extra logic there  In my own application for example  I update a dates when other fields are changed    Class myModel models Model        name   models CharField       date created   models DateField        def save self           if not self pk                  we have a newly created object  as the db id is not set            self date created   datetime datetime now           super myModel   self  save

User · Answer

save   method can be used to insert new record and update existing record and generally used for saving instance of single record row in mysql  in database   update   is not used to insert records and can be used to update multiple records rows in mysql  in database

User · Answer

Both looks similar  but there are some key points    save   will trigger any overridden Model save   method  but update   will not trigger this and make a direct update on the database level  So if you have some models with overridden save methods  you must either avoid using update or find another way to do whatever you are doing on that overridden save   methods  obj save   may have some side effects if you are not careful  You retrieve the object with get       and all model field values are passed to your obj  When you call obj save    django will save the current object state to record  So if some changes happens between get   and save   by some other process  then those changes will be lost  use save update fields          for avoiding such problems  Before Django version 1 5  Django was executing a SELECT before INSERT UPDATE  so it costs 2 query execution  With version 1 5  that method is deprecated    In here  there is a good guide or save   and update   methods and how they are executed

User · Answer

Update only works on updating querysets  If you want to update multiple fields at the same time  say from a dict for a single object instance you can do something like   obj   dict   update your dict  obj save     Bear in mind that your dictionary will have to contain the correct mapping where the keys need to be your field names and the values the values you want to insert

User · Answer

There are several key differences   update is used on a queryset  so it is possible to update multiple objects at once   As  FallenAngel pointed out  there are differences in how custom save   method triggers  but it is also important to keep in mind signals and ModelManagers  I have build a small testing app to show some valuable differencies  I am using Python 2 7 5  Django  1 7 7 and SQLite  note that the final SQLs may vary on different versions of Django and different database engines   Ok  here s the example code   models py   from   future   import print function from django db import models from django db models import signals from django db models signals import pre save  post save from django dispatch import receiver    author      sobolevn   class CustomManager models Manager       def get queryset self           super query   super models Manager  self  get queryset           print  Manager is called   super query          return super query   class ExtraObject models Model       name   models CharField max length 30       def   unicode   self           return self name   class TestModel models Model        name   models CharField max length 30      key   models ForeignKey  ExtraObject       many   models ManyToManyField  ExtraObject   related name  extras        objects   CustomManager        def save self   args    kwargs           print  save   is called            super TestModel  self  save  args    kwargs       def   unicode   self             Never do such things  access by foreing key  in real life            because it hits the database          return u           format self name  self key name  self many count       receiver pre save  sender TestModel   receiver post save  sender TestModel  def reicever  args    kwargs       print  signal dispatched     views py   def index request       if request and request method     GET            from models import ExtraObject  TestModel            Create exmple data if table is empty          if TestModel objects count      0              for i in range 15                   extra   ExtraObject objects create name str i                   test   TestModel objects create key extra  name  test  d    i                  test many add test                  print test          to edit   TestModel objects get id 1          to edit name    edited test          to edit key   ExtraObject objects create name  new for           to edit save            new key   ExtraObject objects create name  new for update           to update   TestModel objects filter id 2  update name  updated name   key new key            return any kind of HttpResponse   That resuled in these SQL queries     to edit   TestModel objects get id 1   QUERY   u SELECT  main testmodel   id    main testmodel   name    main testmodel   key id   FROM  main testmodel   WHERE  main testmodel   id     s LIMIT 21   - PARAMS    u 1       to edit save    QUERY   u UPDATE  main testmodel  SET  name     s   key id     s  WHERE  main testmodel   id     s   - PARAMS    u  edited test    u 2   u 1      to update   TestModel objects filter id 2  update name  updated name   key new key   QUERY   u UPDATE  main testmodel  SET  name     s   key id     s  WHERE  main testmodel   id     s   - PARAMS    u  updated name    u 3   u 2     We have just one query for update   and two for save     Next  lets talk about overriding save   method  It is called only once for save   method obviously  It is worth mentioning  that  objects create   also calls save   method    But update   does not call save   on models  And if no save   method is called for update    so the signals are not triggered either  Output   Starting development server at http   127 0 0 1 8000  Quit the server with CONTROL-C     TestModel objects get id 1   Manager is called   lt TestModel  edited test new for 0 gt   Manager is called   lt TestModel  edited test new for 0 gt   save   is called  signal dispatched signal dispatched    to update   TestModel objects filter id 2  update name  updated name   key new key   Manager is called   lt TestModel  edited test new for 0 gt     As you can see save   triggers Manager s get queryset   twice  When update   only once    Resolution  If you need to  silently  update your values  without save   been called - use update  Usecases  last seen user s field  When you need to update your model properly use save

[django] Django: save() vs update() to update the database?

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