Auto-populate field based on reference selection: "Cannot assign to read only property" error

I understand your issue! You're running into a fundamental limitation with validation rules - validation rules are meant for validating data, not for modifying it. The doc object you receive in a validation rule is read-only, which is why you're getting that error when trying to assign to doc.parent.listingElements.

The good news is that what you're trying to do (populate one field based on another) is absolutely possible in Sanity, but you need to use a custom input component instead of a validation rule.

Here's the approach:

Create a custom input component that:

1. Watches the first field's value (the reference selection)
2. Makes your request when it changes
3. Updates the second field (listingElements) using the onChange prop with proper patch events

Here's a basic example structure:

import { set, unset } from 'sanity'
import { useEffect } from 'react'
import { useFormValue } from 'sanity'

export function ListingElementsInput(props) {
  const { onChange, value } = props
  
  // Get the value of the first field (adjust path as needed)
  const referenceField = useFormValue(['yourReferenceFieldName'])
  
  useEffect(() => {
    if (referenceField) {
      // Make your request here
      fetchProductRefs(referenceField).then(productRefs => {
        // Update the field value
        onChange(set(productRefs))
      })
    }
  }, [referenceField])
  
  // Render your field UI (or use renderDefault if you want the standard UI)
  return props.renderDefault(props)
}

Then in your schema:

{
  name: 'listingElements',
  type: 'array', // or whatever type
  components: {
    input: ListingElementsInput
  }
}

The key differences from validation:

• Custom input components have write access via the onChange prop
• You can use useFormValue() to watch other fields in the document
• You can make async requests and update the field accordingly

Check out this guide on creating custom input components for more details on the implementation.

Why validation rules don't work for this: Validation rules are purely for checking if data is valid - they run after changes and return errors/warnings. They're intentionally read-only to prevent side effects during validation. Custom input components are the proper extension point for conditional field population and dynamic behavior.