Dynamic Lookup with MongoDB Aggregation API

In Mongoose, there's a great and useful feature called Population, which makes it possible to dynamically connect a field to another document from another collection.

Let's say, in sales document you have costumer field. Costumer might be a person or a company. And you want to keep costumer in one field. No problem. You can create a schema like the one below.

costumer: { type: ObjectId, refPath: "costumerType" },
costumerType: { type: String, enum: ["people", "companies"] }

Then, when you need to get this sales document, you can "populate" costumer field like below.

await sales.find({}).populate("costumer")

Mongoose will bring you the costumer, whether person or company.
Very useful.

Switching to Aggregation API

If you're building much bigger app, there might be a need for complex database queries. MongoDB have a great query API called Aggregation API.

At the point that you're working with Aggregation API, you may encounter the limitation that aggregation doesn't directly support dynamic lookups — lookup ($lookup) is an operator for bringing documents from other collections in same database.

Nevertheless, nothing is impossible.

I created a trick to make dynamic lookups possible by the following method.

await sales.aggregate([
  // Lookup for people and get result into "_temporaryPeople"
  { $lookup: {
    from: "people",
    localField: "costumer",
    foreignField: "_id",
    as: "_temporaryPeople",
  }},
  // Lookup for companies and get result into "_temporaryCompanies"
  { $lookup: {
    from: "companies",
    localField: "costumer",
    foreignField: "_id",
    as: "_temporaryCompanies",
  }},
  // Set "costumer" field with merge of two variables.
  // This will set "costumer" the wanted document.
  { $set: {
    costumer: {
     $mergeObjects: [
       { $first: "$_temporaryPeople" },
       { $first: "$_temporaryCompanies" },
     ]
    }
  }},
  // After we're done, we can destroy temporary fields.
  { $unset: ["_temporaryPeople", "_temporaryCompanies"] },
])

Until MongoDB adds dynamic lookup to the Aggregation API,
I think this is the easiest way to handle this issue.

Stay with MongoDB 🏄‍♂️