MongoDB $graphLookup

Follow a relationship recursively through one collection.

Start with one value, match it against `connectToField`, then reuse `connectFromField` from every match to continue traversing. Set a maximum depth when the domain has a known bound.

Stages are the top-level steps in a pipeline. Each stage receives documents, changes the stream in one specific way, and passes its output to the next stage.

Course use: Beyond Hard Mode.

{ $graphLookup: {
  from: "categories",
  startWith: "$parentId",
  connectFromField: "parentId",
  connectToField: "_id",
  as: "ancestors"
} }