An `isUnique` function for unique slugs across a schema type but the same across a base document

i18n-document-level-unique-slugs.js

{
  name: 'slug',
  title: 'Slug',
  type: 'slug',
  options: {
    source: 'title',
    isUnique: async (slug, options) => {
      const {document: sanityDocument} = options

      // Filter out additional parts of the _id to get the original
      const baseId = sanityDocument._id
        .split(`.`) // Replace with your delimeter if not using subpath
        .filter((idPart) => ![`drafts`, `i18n`, sanityDocument.__i18n_lang].includes(idPart))
        .join(`.`)
      const type = sanityDocument._type

      // 1. Check all documents of this same _type
      // 2. With this same slug
      // 3. But not with this same base _id
      const query = `*[_type == $type && slug.current == $slug && !(_id match $baseId)]`
      const params = {slug, type, baseId}
      const matchingSlugs = await client.fetch(query, params)

      return !matchingSlugs.length
    },
  },
},