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The ".save()" method is used to write a model instance to the database. It can be an existing record or even a new one. For an existing record, Django will run a SQL UPDATE statement on the database. For a new record, Django will run an INSERT.
When updating an existing record, Django will update every field. That means, if you changed only one field on the model instance, all the fields will be updated (with the values staying the same). This is inefficient, especially if the model has many fields.
Another problem is that if there's a significant amount of time in between the fetching of the record and the saving of it, it is possible that another request will have updated one of the fields on that record, and your model instance still has the old value.
Calling ".save()" now will overwrite the new value that someone else saved.
To prevent updating every field of the model you can use the update_fields parameter. It expects a list of fields that you want to save. The UPDATE that is run against the database then only updates those fields that you have specified.
For example, in case we only want to update the price of a toy, we would run "toy.save(update_fields=["price"])".
The ".update()" method is available on each queryset object of the ORM. For example, "Toy.objects.filter(price__lt=50).update(price=50)".
This would update the price of all toys that are below a certain price. While this is for a range of records, it could also be used to update a single record: "Toy.objects.filter(name="hippo").update(price=120)".
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