Custom validation rules based on another field's value

Great question! You can definitely use rule.custom() to create conditional validation based on another field's value. Here's how to do it:

The key is accessing the context parameter in your custom validation function, which gives you access to the parent document and other fields. Here's a practical example:

defineField({
  name: 'u',
  title: 'U Field',
  type: 'string',
  validation: (rule) =>
    rule.custom((value, context) => {
      // Check if field X has a specific value
      const xValue = context?.document?.x;
      
      if (xValue === 'someSpecificValue') {
        // Make U field required when X is set to this value
        if (!value) {
          return 'U field is required when X is set to "someSpecificValue"';
        }
      }
      
      return true; // Valid
    }),
})

Some important things to note:

Accessing other fields: Use context?.document?.fieldName to access sibling fields at the document level, or context?.parent?.fieldName for fields within the same object/array.

Return values: Your custom function should return:

• true when the value is valid
• An error message string when invalid
• You can also return an object like {message: 'Error', level: 'error'} for more control

Handling undefined: Always check if values exist since fields might be empty, especially during initial document creation.

Here's a more complete example with different validation levels:

validation: (rule) =>
  rule.custom((value, context) => {
    const x = context?.document?.x;
    
    if (x === 'critical') {
      if (!value) {
        return 'U is required when X is critical'; // Error (blocks publishing)
      }
    } else if (x === 'important') {
      if (!value) {
        return { 
          message: 'Consider filling U when X is important',
          level: 'warning' // Warning (allows publishing)
        };
      }
    }
    
    return true;
  }),

You can find more details about custom validation in the official validation documentation and this specific community answer about conditional validation.

Remember that validation only runs in the Studio interface - API mutations bypass these rules, so implement server-side validation if you're accepting content through other channels!