How to prevent self-reference in child-parent setup in Sanity using custom validation or filter option.

Great question! Preventing self-references in parent/child relationships is a common need. The good news is you can use the filter option on your reference field to exclude the current document.

Here's how to prevent a category from referencing itself as a subcategory:

{
  name: 'category',
  type: 'document',
  fields: [
    {
      name: 'title',
      type: 'string'
    },
    {
      name: 'parentCategory',
      type: 'reference',
      to: [{type: 'category'}],
      options: {
        filter: ({document}) => {
          return {
            filter: '_id != $id && !(_id in path("drafts.**"))',
            params: {
              id: document._id.replace(/^drafts\./, '')
            }
          }
        }
      }
    }
  ]
}

What's happening here:

1. The filter option takes a function that receives the current document as context
2. We return a GROQ filter that excludes documents where _id matches the current document's ID
3. We use document._id.replace(/^drafts\./, '') to handle both draft and published versions
4. The !(_id in path("drafts.**")) part ensures we're comparing against the published ID

Important notes:

• Always check that document properties exist before using them (the example above assumes _id is always present, which it should be)
• This filter only constrains the search results when selecting a reference—it doesn't validate existing references if you change the logic later
• The filter uses the published document ID for comparison, which is what you want for referential integrity

This pattern works for any self-referential scenario where you need to prevent a document from referencing itself!