Conditional validation of string

Validate a field only if a sibling boolean type is set to true.

conditionalValidation.js

export default {
  name: "docName",
  title: "Document Name",
  type: "document",
  fields: [
    {
      name: 'flag',
      title: 'Flag for Validation',
      type: 'boolean',
    },
    {
      name: 'entry',
      type: 'string',
      title: 'Entry',
      validation: Rule => Rule.custom((field, context) => (context.document.flag && field === undefined) ? "This field must not be empty." : true),
    },
  ],
};

There may be cases where a field should only be validated when flagged. This short snippet will require a non-empty field entry only when flag is set to true.

If flag is set to false, entry will not be validated (i.e., the string can be empty).