Since allow_null=True, the serializer can still validate and partially update even though partial=False

[fbozhang]

Description:

When using Django’s model and serializer as shown below, the FlowSerializer allows partial updates even when partial=False, due to allow_null=True for the flow_id field.

Model:

class Flow(models.Model):
    flow_id = models.IntegerField(null=True)
    flow_info = models.JSONField()

Serializer:

class FlowSerializer(serializers.ModelSerializer):
    flow_id = serializers.IntegerField(allow_null=True, required=False)
    
    class Meta:
        model = Flow
        fields = "__all__"

Input data:

FlowSerializer(instance, data={"flow_info": {"test":1}})

When the input data does not include the flow_id field, the serializer still passes validation, resulting in a partial update. This behavior is unexpected because partial=True is not used.

Source Code Insight:
The relevant code in the Django REST framework source is:

    def get_default(self):
        """
        Return the default value to use when validating data if no input
        is provided for this field.

        If a default has not been set for this field then this will simply
        raise `SkipField`, indicating that no value should be set in the
        validated data for this field.
        """
        if self.default is empty or getattr(self.root, 'partial', False):
            # No default, or this is a partial update.
            raise SkipField()

Questions:

• Why does the get_default method check getattr(self.root, 'partial', False)?
• Why does the serializer allow validation to pass in non-partial update scenarios, even when not all fields are provided?
• Shouldn’t the serializer require all fields to be provided when partial=False?

This behavior is causing confusion and leads to unintended partial updates. Any insights or clarification on this design choice would be greatly appreciated.

[Himalaypatel75]

you are passing required=False in FlowSerializer . don't you think you should pass required=True for desired result ? for failing validation ?
@fbozhang

[fbozhang]

you are passing required=False in FlowSerializer . don't you think you should pass required=True for desired result ? for failing validation ? @fbozhang
I don't think it's necessary to make it required. Because it allows nulls, it supports non-required fields, and if I use required=True, it can't be modified locally. I use flow_id = serializers.IntegerField(allow_null=True, required=False) just to express my question more clearly. In fact, when I use serializers.ModelSerializer, this field is originally a non-required parameter.
You can refer to the historical versions of Django and find that get_default is not what I expected at the beginning.From the comments, it can be seen that this is intentional. I want to know what went wrong at that time that led to such a change.
@Himalaypatel75

[Shekhrozx]

you are passing required=False in FlowSerializer . don't you think you should pass required=True for desired result ? for failing validation ? @fbozhang
I don't think it's necessary to make it required. Because it allows nulls, it supports non-required fields, and if I use required=True, it can't be modified locally. I use flow_id = serializers.IntegerField(allow_null=True, required=False) just to express my question more clearly. In fact, when I use serializers.ModelSerializer, this field is originally a non-required parameter.
You can refer to the historical versions of Django and find that get_default is not what I expected at the beginning.From the comments, it can be seen that this is intentional. I want to know what went wrong at that time that led to such a change.
@Himalaypatel75

Understanding the Behavior:

• get_default Method and partial Check:

  • The get_default method checks if the partial attribute of the serializer's root (the serializer instance itself) is False. This logic determines whether the serializer is in partial update mode.

  • If partial is False, the method assumes that a complete set of data is expected, and any missing fields will be considered errors.

• Allowing Validation to Pass:

  • The serializer allows validation to pass even when not all fields are provided because the flow_id field has allow_null=True. This means that the field can be null or omitted in the input data without causing validation errors.

  • The combination of partial=False and allow_null=True creates a discrepancy: while partial=False suggests a complete update, allow_null=True permits partial updates by allowing the flow_id field to be omitted.