Subcollection

@available(iOS 13, tvOS 13, macOS 10.15, watchOS 7, *)
public class Subcollection : Pipeline, @unchecked Sendable

A Subcollection is a special type of pipeline constructed for sub-queries.

It is not tied to a primary database instance upfront and cannot be executed directly; instead, it is intended to be converted into an array or scalar expression and used within another pipeline's stage (such as addFields).

// Calculate the average rating of reviews for each book and add it as a field.
firestore.pipeline().collection("books")
  .addFields([
     Subcollection("reviews")
         .aggregate([Field("rating").average().as("avgRating")])
         .toScalarExpression().as("averageRating")
  ])