How do I wait for a promise to fill and then return a generator function?

Question

I know this is wrong, but essentially I want to

• connect to a db/orm via a promise
• wait on that promise to fulfill and get the models (the return from the promise)
• use the results for form a middleware generator function to place the models on the request

I suspect that this isn't the best approach, so essentially I have two questions:

• Should I be rewriting my db/orm connect to a generator function (I have a feeling that is more inline with koa style)
• Back to the original question (as I am sure I will not get a chance to rewrite all my business logic) - how do I wait on a promise to fulfill and to then return a generator function?

This is my poor attempt - which is not working, and honestly I didn't expect it to, but I wanted to start by writing code, to have something to work with to figure this out:

var connectImpl = function() {
  var qPromise = q.nbind(object.method, object);
  return qPromise ; 
}

var domainMiddlewareImpl = function() {
    let connectPromise = connectImpl()
    return connectPromise.then(function(models){
        return function *(next){
            this.request.models = models ;
        }
    })
}

var app = koa()
app.use(domainMiddlewareImpl())

Answer 1

A context sensitive answer based on the info provided by Hugo (thx):

 var connectImpl = function() {
     var qPromise = q.nbind(object.method, object);
     return qPromise ; 
 }


var domainMiddlewareImpl = function () {
    let models = null ;
    return function *(next) {
        if(models == null){
            //////////////////////////////////////////////////////
            // we only want one "connection", if that sets up a 
            //     pool so be it
            //////////////////////////////////////////////////////
            models = yield connectImpl() ;
        }
        this.request.models = models.collections;
        this.request.connections = models.connections;
        yield next
    };
};

My example the connectImpl is setting up domain models in an ORM (waterline for now), connecting to a database (pooled), and returning a promise for the ORM models and DB connections. I only want that to happen once, and then for every request through my Koa middleware, add the objects to the request.

Answer 2

According to this, you can do the following:

var domainMiddlewareImpl = function() {
    return function *(){
        this.request.models = yield connectImpl();
    };
};