how to use F expression to first cast a string to int, then add 1 to it then cast to string and update - django-queryset

Question

I have a DB column which is generic type for some stats(qualitative and quantitative info).
Some values are string - type A and some values are numbers stored as string - type B.
What i want to do is cast the B types to number then add one to them and cast back to string and store.
Metadata.objects.filter(key='EVENT', type='COUNT').update(value=CAST(F(CAST('value', IntegerField()) + 1), CharField())
What i want to do is avoid race conditions using F expression and
update in DB.
https://docs.djangoproject.com/en/4.0/ref/models/expressions/#avoiding-race-conditions-using-f
It says in below post that casting and updating in db is possible for mysql
Mysql Type Casting in Update Query
I also know we can do arithmetic very easily on F expressions as it supports it and we can override functionality of add as well. How to do arthmetic on Django 'F' types?
How can i achieve Cast -> update -> cast -> store in Django queryset?

Answer 1

Try using annotation as follows:
Metadata.objects
.filter(key='EVENT', type='COUNT')
.annotate(int_value=CAST('value', IntegerField()))
.update(value=CAST(F('int_value') + 1, CharField())
Or maybe switching F and CAST works?
Metadata.objects
.filter(key='EVENT', type='COUNT')
.update(value=CAST( # cast the whole expression below
CAST( # cast a value
F('value'), # of field "value"
IntegerField() # to integer
) + 1, # then add 1
CharField() # to char.
)
I've added indentation, it helps sometimes to find the errors.
Also, doc says, CAST accepts field name, not an F-object. Maybe it works without F-object at all?
UPD: switched back to first example, it actually works :)

Answer 2

I believe the answer from #som-1 was informative but not substantiated with info or debugged data. I believe assuming is not always right.
I debugged the mysql queries formed in these two cases -
1 - Metadata.objects.update(value=Cast(Cast(F('value'), output_field=IntegerField()) + 1, output_field=CharField()))
2 - Metadata.objects.update(value=Cast(Cast('value', IntegerField()) + 1, CharField())) and
both give the same output as expected.
UPDATE Metadata SET value = CAST((CAST(value AS signed integer) + 1) AS char) WHERE ( key = 'EVENT' AND type = 'COUNT' )
Please find the link to add mysqld options to my.cnf and debug your queries. Location of my.cnf file on macOS
enabling queries - https://tableplus.com/blog/2018/10/how-to-show-queries-log-in-mysql.html