multiplying two fields

[jose angel encinas ramos]

Guys gd evening

I need help,really i don't kwnow how to do a multiply two fields, have a model called Articles.

class Articles(models.Model):

#models article

Nuevo = 'Nuevo'

Semi_nuevo = 'Semi_nuevo'

Usado = 'Usado'

Otros = 'Otros'

STATE_ACTUAL = (

(Nuevo, 'Nuevo'),

(Semi_nuevo,'Semi_nuevo'),

(Usado,'Usado'),

(Otros,'Otros')

)

name = models.CharField(max_length=50)

quantity = models.PositiveIntegerField()

fk_brand = models.ForeignKey(Brand, null=True, on_delete=models.SET_NULL)

model = models.CharField(max_length=50)

fk_category = models.ForeignKey(Category, null=True, on_delete=models.SET_NULL)

cost_buy = models.DecimalField(max_digits=10, decimal_places=2)

fk_supplier = models.ForeignKey(Supplier, null=True, on_delete=models.SET_NULL)

userful_life = models.DateField()

actual_state = models.CharField(max_length=12, choices=STATE_ACTUAL)

date_check = models.DateField()

location = models.CharField(max_length=50)

img = models.ImageField(upload_to='articles', null=True, blank=True)

description = models.TextField(blank=True)

#actions

created = models.DateTimeField(auto_now_add=True)

update = models.DateTimeField(auto_now=True)

class Meta:

verbose_name = 'Article'

verbose_name_plural = 'Articles'

@property

def result(self,*args, **kwargs):

mult = self.cost_buy * self.quantity

return mult

class Meta:

verbose_name = 'Article'

verbose_name_plural = 'Articles'

def __str__(self):

return '{} {} {}'.format(self.name ,self.cost_buy, self.quantity)

so i want to multiply 2 filed , cost_buy * quantity and the result show in the template

i was to try did this query in view.py 

data = Articles.objects.all().annotate(result=F('coust_buy') * F('quantity')).output_field=FloatField('result')

 

but isn't work

[Ross Meredith]

data = Articles.objects.all().annotate(result=F('coust_buy') * F('quantity')).output_field=FloatField('result'),

contains a type for the field "cost_buy" (i.e. not "coust_by").

If that isn't it please give us the traceback.

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[Ross Meredith]

Apologies, typo myself!

That meant to say "typo" not "type".

[jose angel encinas ramos]

tanks Ross, i fixed my error but now only show me this

>>> data = Articles.objects.all().annotate(F=('cost_buy') * F('quantity')).output_field=FloatField('result')
>>> print(data)
<django.db.models.fields.FloatField>
>>>

what do you think?

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[Ross Meredith]

You need to read the documentation as always - https://docs.djangoproject.com/en/3.1/ref/models/expressions/#using-f-with-annotations

It says that the output_field is a keyword argument for the annotate method.

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[Mai Elshiashi]

This will work 

data = Articles.objects.all().annotate(result=ExpressionWrapper(F('cost_buy') * F('quantity') , output_field=FloatField()))
now data is a queryset. so you can do something like this
data[0].result
data.values("result")