如何设置Django模型字段的NoData值

Question

I just realized that the dataset I uploaded to Django model has a field (call it 'A') with "-999" which indicates NoData. This is throwing off my calculations, but I don't want to totally exclude these objects from my filters, just for calculations, for example: objects.aggregate(Avg('A')).

How do I tell my Django model to treat this value as the NoData for field A? I see null=True tells Django to store empty values as NoData, but what if it should store Null when the values = -999.

If there is not a straightforward way, is the best/most efficient way just to use exclude() whenever I make these calculations? Or maybe I should do some sort of replace in my database to convert -999 in field A to Null (how?). I thank you

Answer 1

A quick solution would be to use exclude in your aggregate queries:

YourModel.objects.exclude(a=-999).aggregate(Avg('a'))

If you wanted to replace the -999 values with None :

1. Adjust your Model field with the correct parameters (eg: null=True)
2. Do a schema migration
3. Do a Data Migration

The data migration would need to call a function something like:

def update_nodata(apps, schema_editor):
    YourModel = apps.get_model('yourappname', 'YourModel')
    for obj in YourModel.objects.filter(a=-999):
        obj.a = None
        obj.save()