new-validation-rule

To add a custom server-side validation rule in a Rust backend, implement a ValidationRule struct and register it at app bootstrap with a unique ValidationRuleId to enforce domain-specific constraints on request DTOs.

To perform database lookups during API request DTO validation, implement a context-aware ValidationRule that accesses database services via a RuleContext, ensuring single-field checks have the required external dependencies.

You can return i18n-ready error codes from backend validation by exposing a unique ValidationRuleId and returning standardized error codes from your custom rule, enabling direct frontend translation of validation failures.

You need a custom ValidationRule instead of built-in validators when your API requires context-aware checks, such as database lookups or service access, to enforce domain-specific constraints beyond standard format validation.

To register a custom validation rule at bootstrap in your backend application, initialize the ValidationRule struct during the app startup sequence, ensuring it is mapped to its unique ValidationRuleId before processing API requests.