Display an array of references as a checklist

    {
      name: 'ReferenceMultiSelect',
      description: 'This will create a multiselect checkbox list of references',
      title: 'Country',
      type: 'array',
      of: [
        {
          type: 'reference',
          to: { type: 'country' },
        },
      ],
      inputComponent: ReferenceSelect,
    },

import React, { useEffect, useState } from 'react';
import { Card, Flex, Checkbox, Box, Text } from '@sanity/ui';
import { FormField } from '@sanity/base/components';
import PatchEvent, { set, unset } from '@sanity/form-builder/PatchEvent';
import { useId } from '@reach/auto-id';
import client from 'part:@sanity/base/client';

const studioClient = client.withConfig({ apiVersion: '2021-10-21' });

const ReferenceSelect = React.forwardRef((props, ref) => {
  const [countries, setCountries] = useState([]);

  useEffect(() => {
    const fetchCountries = async () => {
      await studioClient
        .fetch(
          `*[_type == 'country']{
          _id,
          title
        }`
        )
        .then(setCountries);
    };

    fetchCountries();
  }, []);

  const {
    type, // Schema information
    value, // Current field value
    readOnly, // Boolean if field is not editable
    markers, // Markers including validation rules
    presence, // Presence information for collaborative avatars
    compareValue, // Value to check for "edited" functionality
    onFocus, // Method to handle focus state
    onBlur, // Method to handle blur state
    onChange, // Method to handle patch events,
  } = props;

  const handleClick = React.useCallback(
    (e) => {
      const inputValue = {
        _key: e.target.value.slice(0, 10),
        _type: 'reference',
        _ref: e.target.value,
      };

      if (value) {
        if (value.some((country) => country._ref === inputValue._ref)) {
          onChange(
            PatchEvent.from(
              set(value.filter((item) => item._ref != inputValue._ref))
            )
          );
        } else {
          onChange(PatchEvent.from(set([...value, inputValue])));
        }
      } else {
        onChange(PatchEvent.from(set([inputValue])));
      }
    },
    [value]
  );

  const inputId = useId();

  return (
    <FormField
      description={type.description} // Creates description from schema
      title={type.title} // Creates label from schema title
      __unstable_markers={markers} // Handles all markers including validation
      __unstable_presence={presence} // Handles presence avatars
      compareValue={compareValue} // Handles "edited" status
      inputId={inputId} // Allows the label to connect to the input field
      readOnly={readOnly}
    >
      {countries.map(country => (
        <Card padding={2}>
          <Flex align='center'>
            <Checkbox
              id={country._id}
              style={{ display: 'block' }}
              onClick={handleClick}
              value={country._id}
              checked={
                value ? value.some((item) => item._ref === country._id) : false
              }
            />
            <Box flex={1} paddingLeft={3}>
              <Text>
                <label for={country._id}>{country.title}</label>
              </Text>
            </Box>
          </Flex>
        </Card>
      ))}
    </FormField>
  );
});

export default ReferenceSelect;