Breeze client-side custom validation with server-side data

Question

I created a custom validator that check if a username is used on a DB. The whole process of validation works. What is not working is result.

function createExistingUsernameValidator() {
        var name = 'existingUsernameValidator';
        var ctx = { messageTemplate: 'Questa partita I.V.A. o codice fiscale sono già stati inseriti.', displayName: "Partita IVA o Codice Fiscale" };
        var val = new Validator(name, valFunction, ctx);

        return val;

        function valFunction(value, context) {
            var result = ko.observable(true);
            require('services/datacontext').getIsUserByUsername(value, result)
                .then(function () {
                    debugger;
                    return !result();
            });
        }
    }

The promise works: I know because it hits the debbugger line and the retunrnig value is correct.
But the validator always evaluate as false because I'm not returning anything when the validator is called. In other words: it won't wait for the promise.
Is it my bad javascript or something else?
Any help is welcome.
Thank you!

Edited after answer
I've come to a solution that involves Knockout Validation (very useful script).

    function createIsExistingUserKoValidation() {
        ko.validation.rules['existingUsername'] = {
            async: true,
            validator: function (val, params, callback) {
                if (val) {
                    var result = ko.observable();
                    require('services/datacontext').getIsUserByUsername(val, result)
                        .then(function () {
                            callback(!result());
                        });
                }
            },
            message: ' Existing username.'
        };
        ko.validation.registerExtenders();
    }

In the entity creation:

var createDitta = function () {
        var ditta = manager.createEntity(entityNames.ditta,
            {
                id: newGuid(),
                legaleRappresentante: createPersona(),
                isAttiva: true
            });
        ditta.pivaCodFiscale.extend({ existingUsername: { message: ' Existing username.', params: true } });
        ditta.pivaCodFiscale.isValidating(false);

        return ditta;
    };

ditta.pivaCodFiscale.isValidating(false); this is needed because isValidating is initialized with true.

Jay Traband Jay Traband · Accepted Answer · 2013-04-19T17:09:26

The problem is that your valFunction as written will ALWAYS return 'undefined'. ( which is 'falsy'.

The 'return !result()' expression is NOT the return value of 'valFunction', it is simply the result of an anonymous function that executes AFTER valFunction has already returned. This is the async nature of promises.

What you are trying is to write an 'asynchronous' validation which is NOT supported out of the box with Breeze, but the idea IS a good one.

I think that you might be able to accomplish what you want by having your async callback actually 'set' a value on the entity and have that set operation itself trigger a seperate 'synchronous' validation.

This IS a good idea for Breeze to support more naturally so please feel free to add a feature request to the Breeze User Voice for something like "asynchonous validation". We use this to gauge the communities interest in the various proposed features/extensions to Breeze.

Breeze client-side custom validation with server-side data

1 Answers