How do I set a specific value for an attribute of a model every time I save it on Django?

Question

I have a legacy project that saves models with save, bulk_create and other methods within the framework.

What is the best way to set a specific value for an attribute so that every time a record is saved the new value is also saved? This value is constructed based on other attributes of the instance that is being saved.

I pose this question because I'm not sure all ways that save is possible in Django except save and bulk_create and knowing that on bulk_create:

The model’s save() method will not be called, and the pre_save and post_save signals will not be sent.

https://docs.djangoproject.com/en/1.8/ref/models/querysets/#bulk-create

Answer 1

As far as I know, there are 3 ways to create/update model instances (which are records in database tables):

1. Using the model instance method save().
2. Using the queryset methods create(), update(), get_or_create(), update_or_create() and bulk_create().
3. Using raw SQL or other low-level ways.

If you intend to calculate the value of a field when saving, you could override all of the methods I listed above.

Signals (like pre_create) are not a complete solution because they don't get triggered when bulk_create() is used and so some instance could get saved without the calculated attribute.

There is no django way (that I know) to intercept the third point I mentioned (raw SQL).

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You did not elaborate on your use case, but (depending on your table size and change frequency) maybe you could also try:

1. run a periodical process (maybe using crontab) that updates the calculated field of all model instances.
2. add a database trigger that calculates the field.

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Legacy databases or systems or usually not fun to work with, so maybe you will have to settle for a sub-optimal solution.