Node.js - mongoose promises - loop through

Question

I am having a trouble with mongoose used with node.js. In my model I am having only group supervisor's ID, and in this route I want also to add supervisorName to variable (this name is stored in Group model). So, yeah, I've read some about promises, but still, I dont have any idea how to solve that. (Basically, I just want to loop through all groups, get their models from mongodb, and assign supervisor name to every group)

router.get('/manage', function(req, res, next) {
Group.find({}, function(err, groups) {
    groups.forEach(function(group) {
        Group.findById(group.supervisor, function(err, supervisor) {
            group.supervisorName = supervisor.name;
            console.log(supervisor.name);
        });
    });
}).then(function() {
    res.render('groups/groups_manage', {groups : groups});
});
});

One more information, supervisor is another group ;p

Szymek
– Szymek

2018-01-06 17:09:03 +00:00
Commented Jan 6, 2018 at 17:09 — Szymek
– Szymek, Commented Jan 6, 2018 at 17:09

Akshendra Pratap · Accepted Answer · 2018-01-06 17:47:35Z

2

You can map your groups array into array of promises and use Promise.all to resolve them all.

router.get('/manage', function(req, res, next) {
    Group.find({})
    .exec() // 1
    .then(groups => {
        return Promise.all(groups.map(group => { // 2, 3
            return Group.findById(group.supervisor)
            .exec()
            .then((supervisor) => { // 4
                group.supervisorName = supervisor.name;
                return group;
            });
        }));
    })
    .then(propGroups => {
        res.render('groups/groups_manage', {groups : popGroups});
    });
});

Explanations:

Mongoose Model methods supports both callbacks and promises interface. However to use the promise interface we need to use the exec method and remove the callbacks.
Promise.all takes an array of promises. It waits for all of them to resolve and any of them to reject (give error). Note: The promises are executed in parallel
We have an array of group objects, we map them into promises. Calling .findById and again using exec to get a promise from it. Note: Remember to return your promises
We update the object, and return it as the final result of promise.

edited Jan 6, 2018 at 17:47

answered Jan 6, 2018 at 17:20

Akshendra Pratap

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2 Comments

Szymek Over a year ago

Woah, you made my day, can you explain why i need to do this that way? (Any comments on code or something please?)

Tomblarom Over a year ago

Would you please add how to handle two operations? For example findById and counting how often this element is referenced in another collection, through OtherModel.countDocuments?

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