# A-la-carte

You can also define constraints a-la-carte. Meaning you can create them on the fly or store them as JSON or somewhere in a service. As long as it is a struct of constraints, that's all the validation methods accept via the `constraints` argument.

In this sample we validate the public request context `rc`. This sample validates all fields in the `rc`. If you need more control you can specify the `fields` parameter (default all) or the `includeFields` and `excludeFields` parameters in your `validate()` call.

```javascript
// sample REST API create user
    function create( event, rc, prc ){
        var validationResult = validate(
            target      = rc,
            constraints = {
                username : { required : true },
                email    : { required : true, type : "email" },
                password : { required : true }
            }
        )
        if ( !validationResult.hasErrors() ) {
            UserService.createUser( rc.username, rc.email, rc.password );
            prc.response.setData( UserService.readUser( username = rc.username ) );
        } else {
            prc.response
                .setError( true )
                .addMessage( validationResult.getAllErrors() )
                .setStatusCode( STATUS.BAD_REQUEST )
                .setStatusText( "Validation error" );
        }
    }
```