How to subtract two fields in django templates?

Question

Before marking as duplicate to numerous other questions on SO, read. I have three fields, Credit Available, Credit Limit and Credit Balance. I'm getting the values for Credit Limit and Credit Balance from my database. To display Credit Available, I want to do Credit Limit-Credit Balance. I tried doing this from other answers on stackoverflow, but they did not work

<tr>
    <td style="text-align:center">{{ credit.credit_limit|add:"-{{credit.credit_balance}}" }}</td>
    <td align="center">{{ credit.credit_limit}}</td>
    <td align="center">{{ credit.credit_balance }}</td>
</tr>

Can I do this without writing new template tags or using the mathfilter module?

Answer 1

The correct syntax for adding the limit to the balance is

{{ credit.credit_limit|add:credit.credit_balance }}

as per https://docs.djangoproject.com/en/1.10/ref/templates/builtins/#add. Unfortunately, putting a minus sign in front of a variable won't work to make it negative.

You could use a custom template filter, which would be fairly straightforward. It might look like this:

@register.filter(name='subtract')
def subtract(value, arg):
    return value - arg

Then in your template (after loading it) it would be:

{{ credit.credit_limit|subtract:credit.credit_balance }}