Custom validation method

[mhdemd]

Describe the bug
When using a custom validation method validate_price in a serializer to enforce specific validation rules for a DecimalField, the default validation messages from Django are shown instead of the custom validation messages provided in the method.

To Reproduce
Steps to reproduce the behavior:

1. Create a serializer with a DecimalField and a custom validation method for this field.
2. Trigger validation errors for the field.
3. Observe that the default validation messages from Django are shown instead of the custom messages.

Expected behavior
The custom validation messages defined in the validate_price method should be shown when the validation fails.

Code Snippet

from rest_framework import serializers
from .models import Product

class ProductSerializer(serializers.ModelSerializer):
    class Meta:
        model = Product
        fields = ['id', 'name', 'price']
    
    def validate_price(self, value):
        if value < 5:
            raise serializers.ValidationError("Price must be at least 5.")
        if value.as_tuple().exponent < -1:
            raise serializers.ValidationError("Price cannot have more than 1 decimal places.")
        if len(str(value).replace('.', '')) > 5:
            raise serializers.ValidationError("Price cannot have more than 5 digits in total.")
        return value

# In the code snippet above, when the price is less than 5, the custom error message "Price must be at least 5." is shown correctly.
# However, for the other two validation checks, the default Django validation messages are shown instead of the custom messages.

[agammann]

validate_price runs after the DecimalField has parsed and checked its precision. When input has too many decimal places or digits, the field raises its own error first, so validate_price is never called for that input. You can keep validate_price for the minimum price and set the field's precision error messages where the field is declared:

class ProductSerializer(serializers.ModelSerializer):
    price = serializers.DecimalField(
        max_digits=5,
        decimal_places=1,
        error_messages={
            "max_digits": "Price cannot have more than 5 digits in total.",
            "max_decimal_places": "Price cannot have more than 1 decimal place.",
        },
    )

    class Meta:
        model = Product
        fields = ["id", "name", "price"]

    def validate_price(self, value):
        if value < 5:
            raise serializers.ValidationError("Price must be at least 5.")
        return value

Keep max_digits and decimal_places aligned with your model. With DRF 3.18.3 and Django 6.1.2, I tested 4.0, 5.55, and 123456.0: the first reached validate_price, while the other two produced max_decimal_places and max_digits field errors; the custom messages replaced those defaults. The DRF DecimalField docs describe the precision options.