StructBlock StreamField Validation

custom_streamfield.py

from django.core.exceptions import ValidationError
from django.forms.utils import ErrorList

from wagtail.core import blocks


class CTABlock(blocks.StructBlock):
    """A simple call to action section."""

    button_url = blocks.URLBlock(required=False)
    button_page = blocks.PageChooserBlock(required=False)

    class Meta:  # noqa
        template = "streams/cta_block.html"
        icon = "placeholder"
        label = "Call to Action"

    def clean(self, value):
        # Error container 
        errors = {}

        # Check if both fields are empty 
        if not value.get('button_url') and value.get('button_page') is None:
            errors['button_url'] = ErrorList(['You must select a page, or enter a URL. Please fill just one of these.'])
            errors['button_page'] = ErrorList(['You must select a page, or enter a URL. Please fill just one of these.'])
        # Check if both fields are filled
        elif value.get('button_url') and value.get('button_page'):
            errors['button_url'] = ErrorList(['Please select a page OR enter a URL (choose one)'])
            errors['button_page'] = ErrorList(['Please select a page OR enter a URL (choose one)'])

        # If there are errors, raise a validation with the errors dict from line 20. 
        if errors:
            raise ValidationError('Validation error in StructBlock', params=errors)

        # If no ValidationError was raised, perform a regular clean on this StreaemField. 
        return super().clean(value)

As usual, a brilliant and clear tutorial. Thanks so much for it!

Also, it would be great if you can explain how to do something similar but with a StructBlock inside a ListBlock. It's something I have spent a lot of time and I still can't quite find how to do it. Something like

document_item = ListBlock(StructBlock([
        ('doc_name', CharBlock(label="Document name", required=False)),
        ('doc_file', DocumentChooserBlock(label="File to upload", required=False)),
        ('doc_alternative_url', URLBlock(label="Alternative link to the document", required=False)),
    ]))

I tried with something like this:

def clean(self, value):
        
        # This is for forcing the user to choose either an internal link or an external one,
        # but not both.

        # Error container 
        errors = {}

        added_content = value.get('document_item')

        for content_item in added_content:

            # Check if both fields are empty
            if not content_item.get('doc_file') and content_item.get('doc_alternative_url') is "":
                errors['doc_file'] = ErrorList(['You must select a page, or enter a URL. Please fill just one of these.'])
                errors['doc_alternative_url'] = ErrorList(['You must select a page, or enter a URL. Please fill just one of these.'])
            # Check if both fields are filled
            elif content_item.get('doc_file') and content_item.get('doc_alternative_url'):
                errors['doc_file'] = ErrorList(['Please select a page OR enter a URL (choose one)'])
                errors['doc_alternative_url'] = ErrorList(['Please select a page OR enter a URL (choose one)'])
        
        # If there are errors, raise a validation with the errors dict.
        if any (errors):
            raise ValidationError('Validation error in StructBlock', params=errors)
        # If no ValidationError was raised, perform a regular clean on this StreamField.
        return super().clean(value)

But it's clearly not the way to do it... I can prevent the page from being saved, but I can't display the reason or indicate which fields are not correct...

OK, I sorted it. In case you're curious about my approach, I simply took the StructBlock out of the ListBlock:

class DocumentItemBlock(StructBlock):
    doc_name= CharBlock(label="Document name", required=False)
    doc_file= DocumentChooserBlock(label="File to upload", required=False)
    doc_alternative_url= URLBlock(label="Alternative link to the document", required=False)

    def clean(self, value):
        
        # This is for forcing the user to choose either an internal link or an external one,
        # but not both.

        # Error container 
        errors = {}
        
        # Check if both fields are empty
        if not value.get('doc_file') and value.get('doc_alternative_url') is "":
            errors['doc_file'] = ErrorList(['You must select a page, or enter a URL. Please fill just one of these.'])
            errors['doc_alternative_url'] = ErrorList(['You must select a page, or enter a URL. Please fill just one of these.'])
        # Check if both fields are filled
        elif value.get('doc_file') and value.get('doc_alternative_url'):
            errors['doc_file'] = ErrorList(['Please select a page OR enter a URL (choose one)'])
            errors['doc_alternative_url'] = ErrorList(['Please select a page OR enter a URL (choose one)'])
        
        # If there are errors, raise a validation with the errors dict.
        if any (errors):
            raise ValidationError('Validation error in StructBlock', params=errors)
        # If no ValidationError was raised, perform a regular clean on this StreamField.
        return super().clean(value)

And then is just a matter of calling it from the other class:

class DocumentsBlock(StructBlock):
    doc_category = CharBlock(label="Documents group")
    document_item = ListBlock(
        DocumentItemBlock(
            label="Document description", 
            required=False)
    )

Once again, thank you so much for your blog :D

For anyone who's errors are not showing at the field level, try changing your exception on line 33 to:
raise StructBlockValidationError(errors)

This is imported from:
from wagtail.core.blocks.struct_block import StructBlockValidationError

This comment outlines the reason for this. 🙂

Extending stevedya's answer, if someone is not able to import StructBlockValidationError,
try importing from this
from wagtail.blocks.struct_block import StructBlockValidationError