Nsryjdtyk

I'm trying to conditionally change field value during lookup - I have some specific order in mind and I do not want to overwrite field value, just to sort it my way. Let's say, I have classProduct and every class object has product_code field. Now I want to get less than or equal, but it's not trivial - product_code is for most of the time like this A01, B02 and so on and Django lookup lte would work. But now I have fields 0001C01 which I would like to be the biggest value. So during lookup I would like to add 0000 at the begining of every string that does not have this prefix, so it would look like 0001C01, 0000B02, 0000A01.

This sounds fairly straightforward. Fetch the desired Product objects, and for each one, prepend 0000 to product_code if it doesn't start with that string.

products = Product.objects.filter(some_query_expression)

for product in products:

    if not product.product_code.startswith('0000'):

        product.product_code = '0000' + product.product_code

It's not clear if you want to save this value back to the database, or just use it for temporary comparisons. If you do want to save it, call product.save().

You can conditionally annotate your queryset in order to get a new field that has de desired value and then use this field in your filter or order_by clause. For example you could do the following:

from django.db.models import CharField, Value as V, F, Q, Case, When

from django.db.models.functions import Concat



Product.objects.annotate(

    new_product_code=Case(

        When(product_code__iregex=r'^[A-Z]+.*',  # If it starts with letters  

             then=Concat(V('0000'), 'product_code', output_field=CharField())  # Then prepend four 0's

        ), 

        default=F('product_code')  # Else, the original value

    )

).filter(new_product_code__lte='whatever you like')  # Now filter by using your new value

Relevant parts of the documentation are conditional expressions, database functions and QuerySet API reference