Using preview values of referenced documents in parent preview

export default {
  name: 'movie',
  type: 'document',
  fields: [...],
  preview: {
    select: {
      title: 'title',
      director: 'director.name' // if the movie has a director, follow the relation and get the name
    },
    prepare(selection) {
      const {title, director} = selection
      return {
        title: title,
        subtitle: `Directed by: ${director ? director : 'unknown'}`
      }
    }
  }
}

{
  name: 'ticket',
  type: 'document',
  fields: [...],
  preview: {
    select: {
      title: 'title',
      summary: 'summary',
      status: 'status',
    },
    prepare({ title, summary, status }) {
      const EMOJIS = {
        open: '🎫',
        resolved: '✅',
        cancelled: '🚫'
      }
      return {
        title: title,
        subtitle: summary,
        media: <span style={{fontSize: '1.5rem'}}>{status ? EMOJIS[status] : '🎫'}</span>
      }
    }
  }
}