Executing my 3 calls one after the other in my activate function - breeze

Question

I work on a project with durandal/breeze. I have the following code in my activate function:
var activate = function (routeData) {
initLookups();
var idTran = parseInt(routeData.idTran);
var idItin = parseInt(routeData.idItin);
if (idItin == -1)
idItin = datacontext.createItineraryDetailTransport(idTran);
datacontext.getTransportById(idTran, transport);
datacontext.getItineraryById(idItin, itinerary);
}
As you can see in the above code, I have 3 calls to the datacontext:
datacontext.createItineraryDetailTransport >> eventually... if (idItin == -1)
datacontext.getTransportById
datacontext.getItineraryById
The problem right now is that each call is not waiting for the previous one to complete before executing.
My question: how to proceed to be sure one call is finished before executing the next one? Please note that the first call is inside a condition... I'm thinking of using 'promises' but don't know.
Thanks.

Answer 1

The tricky part to what you're trying to do is conditionally chain three calls together.
You can simply chain multiple calls together using the then() method. However in your case, you need an initial promise to chain when the first condition isn't met.
The $.when() method is the trick here, because you can either chain an promise returned by Breeze, or you can chain a "dummy" promise, which is what $.when() gives you. If the first parameter passed to $.when is not a promise, then it returns a promise that is immediately resolved.
If I understand your question correctly, you should be able to write your code something like this:
var activate = function (routeData) {
initLookups();
var idTran = parseInt(routeData.idTran);
var idItin = parseInt(routeData.idItin);
var idtDeferred = $.when();
if (idItin == -1)
idtDeferred = datacontext.createItineraryDetailTransport(idTran);
idtDeferred
.then(datacontext.getTransportById(idTran, transport))
.then(datacontext.getItineraryById(idItin, itinerary));
}
Your code example looks like datacontext.createItineraryDetailTransport is supposed to set the idItin var, but I'm assuming that it returns a promise like typical breeze queries.

Answer 2

Certain parts of your example are unclear to me.
Is initLookups asynchronous? If so, do you have to wait for it to complete before performing the other asynchronous steps?
How can createItineraryDetailTransport be asynchronous when it returns the integer, idItin?
What the heck does createItineraryDetailTransport actually do? My guess is that idItin == -1 when you don't have an ItineraryDetailTransport entity yet and therefore don't have the key you need to call getItineraryById. If so, you have to restructure the signature of createItineraryDetailTransport.
Why does getItineraryById have to wait for getTransportById when they seemingly have nothing in common?
What are transport and itinerary? I'm guessing they are accidentally omitted variables that will be set with the results of the async calls within those datacontext methods.
Where is your error handling? What should happen if one of the async calls fails?
These issues must be sorted out before someone can give you a really good answer.
Joseph Gabriel seems to me to be mostly on the right track although I might have written it a little differently
...
var transport, itinerary;
var promise = (idItin == -1) ?
datacontext.createItineraryDetailTransport(aCallBackThatSets_idItin) :
Q.resolve(); // creates a resolved promise
promise = promise
.then(datacontext.getTransportById(idTran, transport)
.then(datacontext.getItineraryById(idItin, itinerary))
.fail(yourErrorHandler);
return promise; // don't forget to return the promise!
The most important step missing from Joseph Gabriel's suggestion ... and the reason you couldn't make his suggestion work ... is that it neglected to return the promise.
You must return a promise if you want Durandal to wait before activating the view.