How to pass results from one query to another in Robo3T

Question

I'm trying to pass the results from one MongoDB query to another but the results are in Object and I need it as an Array. Example:

var locId = db.getCollection('locations').find({country:"France"}, {_id:1}).toArray()

How can I pass the results to another query:

db.getCollection('products').find({locId: {$in: locId }})

The results from the first query are something like:

array[1] [
obj[0] {_id: LocId123},
obj[1] {_id: LocId456},
obj[1] {_id: LocId789},
]

I was able to pass the values with this:

locId = Array.from(locId , value => value._id )

The problem is that I cannot encapsulate this in an external function because I cannot pass this "_id" by the function parameters. Any ideas on how to automate it?

Answer 1

You can use aggregate() to combine all ids in a array,

• $match country condition
• $group by null means none and push _id in array ids
• $project to show _ids and hide _id

var locId = db.getCollection('locations').aggregate([
  { $match: { country: "france" } },
  {
    $group: {
      _id: null,
      ids: { $push: "$_id" }
    }
  },
  {
    $project: { _id: 0, ids: 1 }
  }
])

Playground: https://mongoplayground.net/p/8_uzjCNMy00

Result:

[{
    "ids": [1, 2, 3]
}]

You Can Access:

db.getCollection('products').find({locId: {$in: locId[0]['ids'] }})

---

If you want to combine both query in single aggregation query then try,

• $match country condition
• $lookup to join products collection using localField _id to foreignField locId
• $unwind to deconstruct products array
• $group by _id null and push all products in products
• $unwind to deconstruct products array
• $project to show required fields

var products = db.getCollection('locations').aggregate([
  {
    $match: { country: "france" }
  },
  {
    $lookup: {
      from: "products",
      as: "products",
      localField: "_id",
      foreignField: "locId"
    }
  },
  { $unwind: "$products" },
  {
    $group: {
      _id: null,
      products: { $push: "$products" }
    }
  },
  { $unwind: "$products" },
  {
    $project: {
      _id: "$products._id",
      locId: "$products.locId"
    }
  }
])

Playground: https://mongoplayground.net/p/xOnohm0OWBV