Filter array to exclude already referenced sections in Sanity document

I can help you filter out already-referenced sections from your reference field! The issue with your current query is the syntax - you're trying to access the parent document's array, but the approach needs adjustment.

Here's the corrected solution:

{
  name: 'section',
  title: 'Section',
  type: 'reference',
  to: { type: 'section' },
  options: {
    filter: ({ document }) => {
      // Get all section IDs already referenced in the primary array
      const referencedIds = document.primary
        ?.map(item => item.section?._ref)
        .filter(Boolean) || []

      return {
        filter: '!(_id in $referencedIds)',
        params: {
          referencedIds
        }
      }
    }
  }
}

What's happening here:

1. Extract referenced IDs in JavaScript: Instead of trying to do this purely in GROQ, we extract the _ref values from the primary array items in the filter function itself. This is more reliable since you have direct access to the document object.

2. Simple GROQ filter: The GROQ filter !(_id in $referencedIds) checks if the current section's _id is NOT in the array of already-referenced IDs.

3. Pass as params: We pass the array of IDs as a parameter to the GROQ query.

Important note: The filter function re-runs when the document changes, so as you add sections to your primary array, they'll automatically disappear from the available options in other items.

Making it reusable:

You can extract this into a utility function to keep your code DRY, as shown in this community example:

// utils/getFilter.js
export const getFilter = (document, field, refField = '_ref') => {
  const existingEntries = document[field]
    ?.map(item => item[refField])
    .filter(Boolean) || []
  
  return {
    filter: '!(_id in $existingEntries)',
    params: {
      existingEntries
    }
  }
}

// Then in your schema:
options: {
  filter: ({ document }) => {
    const referencedIds = document.primary
      ?.map(item => item.section?._ref)
      .filter(Boolean) || []
    
    return {
      filter: '!(_id in $referencedIds)',
      params: { referencedIds }
    }
  }
}

This is a common pattern when building navigation menus, related content pickers, or any scenario where you want to avoid duplicate references in an array!