Migrating schema file to v3 and updating slugify example.

import {FaMap} from "react-icons/fa";
import slugify from "slugify";
import sanityClient from 'part:@sanity/base/client'

function myAsyncSlugifier(input) {
    //const slug = slugify(input);
    /// The type of the reference field to get the value from
    const referenceNameType = 'city'
    /// Query for the referenced "category" and return the "name" field of that referenced document
    const query = `*[_type == "${referenceNameType}" && _id == $nameRef][0] {name}`
    /// Start with the "category: reference id in the draft
    let referenceName = input.cityRef
    /// Fetch the category referenced in this document
    return sanityClient.fetch(query, {nameRef: referenceName}).then(city => {
        /// Set the referenceName to the city.name field
        referenceName = !!city ? city.name : referenceName;
        return slugify(`${referenceName} - ${input.name}`, {lower: true});
    })
}

export default {
    name: 'neighbourhood',
    title: 'Stadtteile',
    type: 'document',
    icon: FaMap,
    fields: [
        {
            name: 'slug',
            title: 'Slug',
            type: 'slug',
            options: {
                source: (doc, options) => ({name: doc.name, cityRef: doc.city._ref}),
                maxLength: 100,
                slugify: myAsyncSlugifier
            },
        },
    ],
    preview: {
        select: {
            title: 'name',
            media: 'image',
            city: 'city.name'
        },
        prepare(selection) {
            const {title, city} = selection
            return {
                title: `${city} - ${title}`,
                //subtitle: date.split('-')[0] // YYYY-MM-DD --> YYYY
            }
        }
    },
}

async function myAsyncSlugifier(input, schemaType, context) {
  const slug = slugify(input)
  const {getClient} = context
  const client = getClient({apiVersion: '2022-12-07'})
  ...
}