I noticed while looking for material for
<https://code.djangoproject.com/ticket/20877> that
<https://docs.djangoproject.com/en/dev/models/instances.html#updating-attributes-based-on-existing-fields/>
mentions that:
product.number_sold = F('number_sold') + 1
is faster than:
product.number_sold += 1
though this doesn't seem to be mentioned in the database optimisation page.
That's easy enough to address, and
<https://docs.djangoproject.com/en/dev/topics/db/optimization.html#do-database-work-in-the-database-rather-than-in-python/>
seems like a sensible place for it.
However the mentions of F() that I did find raised a number of questions.
The F() class seems to be a handy general-purpose way to refer to the value of
a model field..
Firstly, it's not explained how, in expressions like:
product.number_sold = F('number_sold') + 1
(from
<https://docs.djangoproject.com/en/dev/models/instances.html#updating-attributes-based-on-existing-fields/>)
Django knows that F('number_sold') is refers to the product model.
Does it know because product.number_sold is the field that this expression
refers to? What would happen if we did:
product.number_in_stock = F('number_in_stock') - F('number_sold)
(i.e. can we make such calculations multiple other fields in one go?), or:
product.number_to_reorder = F('number_sold)
for example? What are the rules of the usage syntax of F()?
Secondly, the main documentation for F()
<https://docs.djangoproject.com/en/dev/topics/db/queries.html#query-expressions/>
doesn't mention this kind of use at all: it only suggests that it might be
useful in queries.
Since this use seems to be just one of multiple uses for F(), shouldn't a more
general description of F() belong somewhere else (where) instead?
Finally, are there any other useful ways to use F() not covered by these two
examples?
Daniele
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