Sanity Custom Preview Component to get Parent Document

import PreviewComponent from "../components/PreviewComponent";

export default {
  name: "steps",
  title: "Steps",
  type: "object",
  fields: [
    {
      name: "items",
      title: "Add Steps",
      type: "array",
      of: [
        {
          type: "object",
          fields: [
            {
              name: "title",
              title: "Title",
              type: "string"
            },
            {
              name: "description",
              title: "Description",
              rows: 3,
              type: "text"
            }
          ],
          preview: {
            select: {
              _key: "_key",
              title: "title",
              subtitle: "description"
            },
            prepare({ _key, title, subtitle }) {
              return { _key, title, subtitle };
            },
            component: PreviewComponent
          }
        }
      ]
    },
  ],
};

import React from "react";
import { withDocument } from "part:@sanity/form-builder";
import Preview from "part:@sanity/base/preview";

const PreviewComponent = ({ document, value }) => {
  const { steps } = document;
  const { _key, title, subtitle } = value;
  const index = steps.items.findIndex(item => item._key == _key);

  return (
    <Preview
      type="image"
      value={{
        title: title,
        subtitle: subtitle,
        media: <span style={{ fontSize: "1.5rem" }}>{index + 1}</span>
      }}
    />
  );
};

export default withDocument(PreviewComponent);