How to select a record and update it with a single queryset in Django

Question

How do I run an update and select statements on the same queryset rather than having to do two queries   - one to select the object - and one to update the object  The equivalent in SQL would be something like   update my table set field 1    some value  where pk field   some value

User · Answer

only in a case in serializer things  you can update in very simple way   my model serializer   MyModelSerializer      instance my model  data validated data  if my model serializer is valid         my model serializer save     only in a case in form things   instance   get object or 404 MyModel  id id  form   MyForm request POST or None  instance instance  if form is valid        form save

User · Answer

Django database objects use the same save   method for creating and changing objects   obj   Product objects get pk pk  obj name    some new value  obj save        How Django knows to UPDATE vs  INSERT   If the object   s primary key attribute is set to a value that evaluates to True  i e   a value   other than None or the empty string   Django executes an UPDATE  If   the object   s primary key attribute is not set or if the UPDATE didn   t   update anything  Django executes an INSERT    Ref   https   docs djangoproject com en 1 9 ref models instances

User · Answer

If you need to set the new value based on the old field value that is do something like  update my table set field 1   field 1   1 where pk field   some value  use query expressions  MyModel objects filter pk some value  update field1 F  field1     1   This will execute update atomically that is using one update request to the database without reading it first

User · Answer

1st method MyTable objects filter pk some value  update field1  some value    2nd Method q   MyModel objects get pk some value  q field1    some value  q save    3rd method By using get object or 404 q   get object or 404 MyModel pk some value  q field1    some value  q save    4th Method if you required if pk some value exist then update it other wise create new one by using update or create  MyModel objects update or create pk some value defaults   field1   some value

User · Answer

This answer compares the above two approaches  If you want to update many objects in a single line  go for     Approach 1 MyModel objects filter field1  Computer   update field2  cool     Otherwise you would have to iterate over the query set and update individual objects    Approach 2     objects   MyModel objects filter field1  Computer   for obj in objects      obj field2    cool      obj save      Approach 1 is faster because  it makes only one database query  compared to approach 2 which makes  n 1  database queries   For n items in the query set  Fist approach makes one db query ie UPDATE  the second one makes two  SELECT and then UPDATE  The tradeoff is that  suppose you have any triggers  like updating updated on or any such related fields  it will not be triggered on direct update ie approach 1  Approach 1 is used on a queryset  so it is possible to update multiple objects at once  not in the case of approach 2

User · Answer

Use the queryset object update method  MyModel objects filter pk some value  update field1  some value

[sql] How to select a record and update it, with a single queryset in Django?

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