Aggregation of different collections in MongoDB

Question

Not sure how to do this, have a background in SQL and it is quite easier there to achieve what I need to do.

Basically I have the collection news which is like this:

const news = [
{id: '123123123', category: [ObjectID(12312312323), ObjectID(987987987)]},
{id: '123123123', category: []},
{id: '123123123', category: [ObjectID(987987987)]}
]

and then I have the collection category which is like this:

const category = [
{name: "123a", _id: ObjectID(123123123}, title:"blah blah" },
{name: "987a", _id: ObjectID(987987987}, title:"blih blah" ,
]

what I need to do is to join both tables and get an aggregation like

[
{name: "987a", title: "blih blah", count: 2},
{name: "123a", title: "blah blah", count: 1}
 
]

So even in MySQL I would get this like something similar to:

SELECT name, title, count(Select count(category) from news) from categories 

But I am not getting it with mongoDb / mongoose.

Answer 1

• $lookup to join news collection, pass _id as localField and category as foreignField
• $size to get total element count of new collection result

db.category.aggregate([
  {
    $lookup: {
      from: "news",
      localField: "_id",
      foreignField: "category",
      as: "count"
    }
  },
  { $set: { count: { $size: "$count" } } }
])

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