inconsistent validation behavior for child serializers

[ozgurakcali]

Steps to reproduce

With the following serializers structure:

class ChildSerializer(serializers.ModelSerializer):
    field_1 = serializers.CharField(required=True)
    field_2 = serializers.CharField(required=True)
    
    def validate(self, attrs):
        if attrs.get('field_2') == 'invalid':
            raise serializers.ValidationError()
        return attrs

class ParentSerializer(serializers.ModelSerializer):
    children = ChildSerializer(many=True, read_only=False)

serializer = ParentSerializer(data={
    'children': [{
            'field_2': 'value'
    }]
})
serializer.is_valid(raise_exception=True) # No validation error raised in this case, even though field_1 is missing in data

serializer = ParentSerializer(data={
    'children': [{
            'field'_1': 'value',
            'field'_2': 'invalid',
    }]
})
serializer.is_valid(raise_exception=True) # Validation error raised in this case, as field_2 value is 'invalid', that makes the condition on **validate** method hold

Expected behavior

Without explicitly calling is_valid method of child serializer from parent serializer, either do not run any validations on child serializer, or run all validations on child serializer.

Actual behavior

When is_valid method is run on parent serializer, if we do not run child serializer's is_valid method explicitly (in validate_children method for example), validations for field_1 being required are not checked, but validate method of ChildSerializer is called.

If we explicitly call is_valid method of ChildSerializer, all validations for ChildSerializer are run.

As they are defined now, serializer is actually invalid for both cases, but a validation error is raised for the second case, but not for the first case.

[ozgurakcali]

A note, observed that this behavior only happens when sending a PATCH request, for POST and PUT validations are run by default for field_1 being required as well.

If forwarding the "partial update" behavior to child serializers on PATCH requests is the desired behavior, than there doesn't seem to be a problem here. But I'm not sure if that would always be the desired behavior.

[PawelMorawian]

If this is the desired behavior it should be mentioned this in the documentation