Django Calling update on a single model instance retrieved by get

Question

I have a function which currently calls Models object get    which returns either 0 or 1 model objects  If it returns 0  I create a new model instance in the except DoesNotExist clause of the function  Otherwise  I would like to update the fields in the pre-existing instance  without creating a new one  I was originally attempting to call  update   on the instance which was found  but  update   seems to be only callable on a QuerySets  How do I get around changing a dozen fields  without calling  filter   and comparing the lengths to know if I have to create or update a pre-existing instance

User · Answer

I am using the following code in such cases   obj  created   Model objects get or create id some id   if not created     resp   It was created  else     resp   OK     obj save

User · Answer

With the advent of Django 1 7  there is now a new update or create QuerySet method  which should do exactly what you want  Just be careful of potential race conditions if uniqueness is not enforced at the database level   Example from the documentation   obj  created   Person objects update or create      first name  John   last name  Lennon       defaults   first name    Bob           The update or create method tries to fetch an object from database   based on the given kwargs  If a match is found  it updates the   fields passed in the defaults dictionary      Pre-Django 1 7   Change the model field values as appropriate  then call  save   to persist the changes   try      obj   Model objects get field value      obj field   new value     obj save   except Model DoesNotExist      obj   Model objects create field new value    do something else with obj if need be

User · Answer

As of Django 1 5  there is an update fields property on model save  eg   obj save update fields   field1    field2          https   docs djangoproject com en dev ref models instances   I prefer this approach because it doesn t create an atomicity problem if you have multiple web app instances changing different parts of a model instance

User · Answer

Here s a mixin that you can mix into any model class which gives each instance an update method   class UpdateMixin object       def update self    kwargs           if self  state adding              raise self DoesNotExist         for field  value in kwargs items                setattr self  field  value          self save update fields kwargs keys      The self  state adding check checks to see if the model is saved to the database  and if not  raises an error    Note  This update method is for when you want to update a model and you know the instance is already saved to the database  directly answering the original question  The built-in update or create method featured in Platinum Azure s answer already covers the other use-case    You would use it like this  after mixing this into your user model    user   request user user update favorite food  ramen     Besides having a nicer API  another advantage to this approach is that it calls the pre save and post save hooks  while still avoiding atomicity issues if another process is updating the same model

User · Answer

As  Nils mentionned  you can use the update fields keyword argument of the save   method to manually specify the fields to update   obj instance   Model objects get field value  obj instance field   new value obj instance field2   new value2  obj instance save update fields   field    field2      The update fields value should be a list of the fields to update as strings   See https   docs djangoproject com en 2 1 ref models instances  specifying-which-fields-to-save

User · Answer

if you want only to update model if exist  without create it    Model objects filter id   223  update field1   2    mysql query   UPDATE  model  SET  field1    2 WHERE  model   id    223

User · Answer

I don t know how good or bad this is  but you can try something like this   try      obj   Model objects get id some id  except Model DoesNotExist      obj   Model objects create   obj   dict   update your fields dict   obj save

[python] Django: Calling .update() on a single model instance retrieved by .get()?

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