How to make aggregate + populate in Mongoose - node.js

Question

I appreciate some help. I'm doing an api rest with express and mongodb (v3.4.4), using mongoose (v4.10.5). I need to do an aggregation operation, but I do not deal with it. I show you some code. The models (it has more properties, but I have left it simple):
const CategoryModel = mongoose.model('Category', new Schema({
slug: { type: String, unique: true, lowercase: true, index: true },
description: String
}));
const MyModel = mongoose.model('MyModel', new Schema({
category: { type: Schema.Types.ObjectId, ref: 'Category' },
other: [{ type: Schema.Types.ObjectId, ref: 'Other' }],
times_count: { type: Number, default: 0 }
}));
Important, I'm interested in populate category field of MyModel, not other field.
Suppose Category and MyModel has certain records well formed. The request:
MyModel.aggregate([
{
$group : {
_id : '$_id',
times: { $sum: '$times_count' }
}
},
{
$limit: 5
}
]).limit(5).exec().then((data) => {
console.log(data);
}).catch((err) => {
console.error(err);
});
data is correct, has 5 records, but not include category. Now, I try with:
MyModel.aggregate([
{
$group : {
_id : '$_id',
times: { $sum: '$times_count' }
}
},
{
$limit: 5
},
{
$lookup: {
from: 'Category', // I tried with 'Categories' and 'categories'
localField: 'category',
foreignField: '_id',
as: 'category'
}
},
{
$unwind: '$category'
}
]).limit(5).exec().then((data) => {
console.log(data);
}).catch((err) => {
console.error(err);
});
Now data is empty. I set mongoose.set('debug', true); and the operations they look right, inclusive the last operation aggregate, but data is empty...
I do not know if I explained well. Obviously there is something that I am not fully understanding. Thanks in advance.

Answer 1

I get the desired records in objs, the problem is that I only come with the _id and times properties, and I need to populate the category.
That's about right since you didn't explicitedly add the stage to join the other collection.
I've tried adding $project to the aggregation after the $ group but nothing.
In simple terms, $project is for including and excluding new fields using one collection, not joining.
You are looking for $lookup which is for joining one collection with another. When you join a new collection, each document will have a new array field containing the "joined" documents from the other collection.
In your case, your new array field will have one document from the other collection, so you probably want to $unwind also.
MyModel.aggregate([
{
$group : {
_id : '$_id',
times: { $sum: '$times_count' },
category: { $first: '$category' }
}
},
/*
{
$limit: 5
},
*/
{
$lookup: {
from: 'Categories',
localField: 'category',
foreignField: '_id',
as: 'category'
}
},
{
$unwind: '$category'
}
]).exec(...);
In terms of your initial problem, try uncommenting the 2nd stage above and not using limit(5) in your first example.