我正在建立一个数据库系统，并试图添加到模型计算字段以及模板。我想将字段自动生成的结果转换为在两个字段上输入的值。

This is my model.py：

class VinanPet(models.Model):
    transaction_Date = models.DateTimeField(auto_now_add=True, auto_now=False, blank=True, null=True)
    entry_Date = models.DateTimeField(null=True)
    branch = models.CharField(max_length=300, null=True)
    product = models.CharField(max_length=300, null=True)
    tank_Opening = models.DecimalField(max_digits=13, decimal_places=3, null=True)
    tank_Closing = models.DecimalField(max_digits=13, decimal_places=3, null=True)
    tank_Difference = models.DecimalField(max_digits=13, decimal_places=3, null=True)
    pump_Opening = models.FloatField(null=True)
    pump_Closing = models.FloatField(null=True)
    pump_Difference = models.DecimalField(max_digits=13, decimal_places=3, null=True)
    price = models.DecimalField(max_digits=13, decimal_places=3, null=True)
    amount = models.DecimalField(max_digits=13, decimal_places=3, null=True)
    expenses = models.DecimalField(max_digits=13, decimal_places=3, null=True)
    bank = models.DecimalField(max_digits=13, decimal_places=3, null=True)
    teller_id = models.IntegerField(null=True)
    teller = models.ImageField(upload_to="teller", null=True)

字符串
这是我的forms.py

from django import forms
import calculation
from django.forms.widgets import NumberInput
from .models import VinanPet

class ManagerForm(forms.ModelForm):
    
    class Meta:
        model = VinanPet
        fields = "__all__"
        widget = {
            'entry_Date': forms.DateInput(attrs={'type': 'date'}),
            'tank_Difference': forms.DecimalField(disabled=True,widget=calculation.FormulaInput('tank_Opening-tank_Closing'))
        }

型
计算字段仅在表单上有效，不在数据库上。