Get details of linked collection inside another collection using populate() - node.js

Question

I am trying to build a system to record timesheets. For each day, an employee can record different hours, against different tasks assigned to them. Every entry in my Timesheet collection contains an array of individual timesheet entries. Sample data/schema for this collection:
[
{
"_id": "5db2c672620ed61854818ccd",
"EmployeeId": "5da9aed442e3070bbd9f7581",
"TimeSheet": [
{
"_id": "5db2c672620ed61854818ccf",
"TaskId": "5db14152e6537a05258bf573",
"Hours": "2.5",
"Remarks": "Test 2.5"
},
{
"_id": "5db2c672620ed61854818cce",
"TaskId": "5db1886ee6537a05258bf575",
"Hours": "11.5",
"Remarks": "Test 11.5"
}
],
"__v": 0
}
]
For the corresponding Task collection, the data resides in the following way -
[
{
"_id": "5db14152e6537a05258bf573",
"EmployeeId": "5da9aed442e3070bbd9f7581",
"ProjectId": "5db141d9e6537a05258bf574",
"TaskName": "Finish the timesheet page",
"TaskDescription": "Write the front-end and back-end code to allow employees to record their timesheets."
},
{
"_id": "5db1886ee6537a05258bf575",
"EmployeeId": "5da9aed442e3070bbd9f7581",
"ProjectId": "5db141d9e6537a05258bf574",
"TaskName": "Learn Populate",
"TaskDescription": "Learn how MongoDB/Mongoose uses the equivalent of SQL joins"
},
{
"_id": "5db27e3ca2445c05255dbad0",
"EmployeeId": "5da9aed442e3070bbd9f7581",
"ProjectId": "5db141d9e6537a05258bf574",
"TaskName": "Timesheet save API",
"TaskDescription": "Code the API to save a timesheet to the database"
}
]
I am trying to get the Task details (TaskName, TaskDescription and others) into every individual task-timesheet entry. For this, I tried using the populate() method in my controller, like so -
exports.findByEmployee = (req, res) => {
TimeSheet.find({ EmployeeId: req.query.EmployeeId })
.then(timesheets => {
timesheets.forEach((ts, ts_index) => {
ts.TimeSheet.forEach((item, index) => {
Task.findById(item.TaskId).populate('TaskId').exec((err, taskDetails) => {
item.TaskDetails = taskDetails;
})
});
})
res.send(timesheets);
}).catch(err => {
res.status(500).send({
message: err.message || "Some error occurred while retrieving timesheets."
});
});
}
However, the API response (for getting all timesheets) does not contain a key called TaskDetails in the individual task-timesheet section. My guess is that because of asynchronous nature of the function calls, the part res.send(timesheets) is getting fired before the section above it has had the time to finish. So my question is, how do I fix this? To summarize, I want the Task details from the Task collection with every individual timesheet item that is linked to the Task by id. Also, is this the correct way to use populate(), or is there a better/simpler/more correct way that will solve my problem?
EDIT:
Someone requested the models, so here's task and timesheet:
const mongoose = require('mongoose');
var ObjectId = mongoose.Schema.Types.ObjectId;
const TaskSchema = mongoose.Schema({
EmployeeId: ObjectId,
ProjectId: ObjectId,
TaskName: String,
TaskDescription: String
}, { collection: 'TASK' });
module.exports = mongoose.model('Task', TaskSchema);
const mongoose = require('mongoose');
var ObjectId = mongoose.Schema.Types.ObjectId;
const TimeSheetSchema = mongoose.Schema({
EmployeeId: ObjectId,
Date: Date,
TimeSheet: [
{
TaskId: {
type: ObjectId,
ref: 'TASK'
},
Hours: String,
Remarks: String
}
]
}, { collection: 'EMPLOYEE_TIMESHEET' });
module.exports = mongoose.model('TimeSheet', TimeSheetSchema);

Answer 1

There are a few things going on here. 1) TaskId doesn't exist in the Task collection so populate doesn't work (and there is no need to use it there) 2) the item object you declare in your ts.TimeSheet.forEach() will not exist outside of that forEach() loop so adding taskDetails to it doesn't accomplish anything because the item object is destroyed when the forEach() loop finishes.
I believe what you want is something like:
exports.findByEmployee = (req, res) => {
try {
// returns just the TimeSheet object from within the TimeSheet collection
// (recommend renaming one of these to avoid confusion!)
TimeSheet.find({ EmployeeId: req.query.EmployeeId }, 'TimeSheet')
// populates TaskId (which I recommend renaming 'Task')
// with the details from the Task collection
.populate('TimeSheet.TaskId')
// executes the query
.exec(timesheets => {
// sends the object once the query has finished executing
res.send(timesheets);
});
} catch (err) {
res.status(500).send({
message: err.message || 'Some error occurred while retrieving timesheets.',
});
}
};
I would highly recommend the MDN "Local Library" Express tutorial for a good introduction to working with MongoDB and Mongoose.