MongoDb nodejs getting count of items belonging to each category-CodePudding

I have 2 collections, Items and Categories, and I'm trying to get a count of how many items are in each category.

An example of what each collection looks like is

Item = [
{
    name: "item1",
    category: ObjectId("123")
},
{
    name: "item2",
    category: ObjectId("456")
}, ...]

Category = [
{
    _id: ObjectId("123"),
    name: "category1"
}, ...]

So far i can count the items of one category using something like this

Item.countDocuments({'category': Category[0].id}).exec(function(err, result) {
    console.log(result)
}
>> 1

My question is how can I do this with all my categories? If anyone could help or point me in the right direction would be much appreciated.

The ideal output would be something like this:

{
    "category1": 1,
    "category2": 3
}

CodePudding user response：

You need to use two aggregation pipelines to achieve result you want.


Item.aggregate([
  {
    $group: {
      _id: "$category", count: { $sum: 1}
    }
  }
])

In the result _id will be the id used to group fields which will be category id.

[
  {
    "_id": ObjectId(123),
    "count": 1
  },
  {
    "_id": ObjectId(456),
    "count": 1
  }
]

Learn more about count aggregation at - https://www.mongodb.com/docs/manual/reference/operator/aggregation/count/