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So I'm just starting out with Django and using it to store forex prices which are represented as 1.21242, 1.20641, etc...

model.py

from django.db import models

# Create your models here.
class ForexPrice(models.Model):
    openPrice = models.DecimalField(max_digits=6, decimal_places=6)
    highPrice = models.DecimalField(max_digits=6, decimal_places=6)
    lowPrice = models.DecimalField(max_digits=6, decimal_places=6)
    closePrice = models.DecimalField(max_digits=6, decimal_places=6)

My Question Is:
How do the max_digits and decimal_places attributes work? I'm using 6 for both fields assuming max_digits=6 will allow values to be stored up to 6 digits ie: 123456.000000 while decimal_palces=6 will support values up to 000000.123456.

Is this assumption correct or am I missing something?

Getting the following error when I save the record to the DB:

A field with precision 6, scale 6 must round to an absolute value less than 1.

Community
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Farhan Ahmad
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  • https://docs.djangoproject.com/en/3.2/ref/models/fields/#decimalfield – iklinac Apr 28 '21 at 20:29
  • @iklinac Thanks for sharing the resource. Yes, I saw their documentation however the example they provided didn't make sense to me until I saw KKAs answer below. – Farhan Ahmad Apr 28 '21 at 20:32

3 Answers3

26

max_digits must be equal, or higher than decimal_places.

If you were to have 123456.654321 you'd have to define max_digits=12, decimal_places=6.

max_digits is INCLUDING decimal_places.

NKSM
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Kim Kakan Andersson
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  • Ahh... I see. That actually makes more sense now. So if I wanted to support something like 9.123456 I would use max_digits=7, decimal_places=6? Correct? – Farhan Ahmad Apr 28 '21 at 20:29
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"max_digits" represents the number of digits all of your numbers.

"decimal_places" represents the number of digits to the right of the comma.

For ex: to store numbers up to 999.99 with a resolution of 2 decimal places, you’d use:

models.DecimalField(..., max_digits=5, decimal_places=2)

halil ibrahim soyman
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0

Here is a simple analysis

For the below expression in your model:

number = models.DecimalField(max_digits=6, decimal_places=2)

Here is the test & result

=> 9999.99 (correct)
=> 99999.99 (error: Ensure that there are no more than 6 digits in total) 
=> 9999.999 (error: Ensure that there are no more than 6 digits in total)
=> 999.999 (error: Ensure that there are no more than 2 decimal places)
=> 99999.9 (error: Ensure that there are no more than 4 digits before the decimal point)

So, the above example shows that if we use max_digits=6, you can enter total 6 digits as a field value.

As we have used decimal_places=2, we have to think that we can store two decimal places as values, and the rest 6-2 = 4 will be the number of digits before the point.

In the case of:

number = models.DecimalField(max_digits=5, decimal_places=2)

We can store two decimal places as values, and the rest of 5-2 = 3 will be the number of digits before the point.

Mukibul Hasan
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