fix($graphLookup): Handle path-based fields on initial match object.

When the selector field is a path, create full object graph for the starting object to match.

src/operators/pipeline/graphLookup.ts

@@ -1,7 +1,7 @@
 import { computeValue, Options, PipelineOperator } from "../../core";
 import { Iterator, Lazy } from "../../lazy";
 import { Any, AnyObject } from "../../types";
-import { flatten, isNil, isString, ValueMap } from "../../util";
+import { flatten, isNil, isString, setValue, ValueMap } from "../../util";
 import { $lookup } from "./lookup";
 
 interface InputExpr {
@@ -57,9 +57,13 @@ export const $graphLookup: PipelineOperator = (
 
   return collection.map((obj: AnyObject) => {
     // initial object to start matching
-    let matches: AnyObject[] = [
-      { [connectFromField]: computeValue(obj, expr.startWith, null, options) }
-    ];
+    const matchObj = {};
+    setValue(
+      matchObj,
+      connectFromField,
+      computeValue(obj, expr.startWith, null, options)
+    );
+    let matches: AnyObject[] = [matchObj];
     let i = -1;
     const map = ValueMap.init<AnyObject, number>(options.hashFunction);
     do {

test/stackoverflow-examples/nested-top-down-hierarchy-with-aggregation.test.ts

@@ -0,0 +1,130 @@
+import { aggregate } from "../support";
+
+// https://stackoverflow.com/a/79174521
+describe("create nested top-down hierarchy using aggregation", () => {
+  it("passes", () => {
+    const relations = [
+      {
+        _id: "p_commerce_metrics_dtst_v1",
+        category: "dataset",
+        dependentOn: {
+          metrics: ["net_booking_count_v1"]
+        }
+      },
+      // Doc 2:
+      {
+        _id: "net_booking_count_v1",
+        category: "metric",
+        dependentOn: {
+          metrics: ["cancelled_booking_count_v1", "gross_booking_count_v1"]
+        }
+      },
+      // Doc 3:
+      {
+        _id: "cancelled_booking_count_v1",
+        category: "metric",
+        dependentOn: {
+          measures: ["hb_cancel_measure_v1"]
+        }
+      },
+      // Doc 4:
+      {
+        _id: "gross_booking_count_v1",
+        category: "metric",
+        dependentOn: {
+          measures: ["hb_booking_measure_v1"]
+        }
+      },
+      // Doc 5 (Not dependentOn any other document _id. Dead End):
+      {
+        _id: "hb_cancel_measure_v1",
+        category: "measure",
+        usedBy: {
+          metrics: ["cancelled_booking_count_v1", "more_metrics"]
+        }
+      },
+      // Doc 6 (Not dependentOn any other document _id. Dead End):
+      {
+        _id: "hb_booking_measure_v1",
+        category: "measure",
+        usedBy: {
+          metrics: ["gross_booking_count_v1", "more_metrics"]
+        }
+      }
+    ];
+
+    const result = aggregate(relations, [
+      {
+        $match: {
+          _id: "p_commerce_metrics_dtst_v1"
+        }
+      },
+      {
+        $graphLookup: {
+          from: relations,
+          startWith: "$dependentOn.metrics",
+          connectFromField: "dependentOn.metrics",
+          connectToField: "_id",
+          depthField: "depth",
+          as: "dependentOnMetrics"
+        }
+      },
+      {
+        $set: {
+          dependentOn: {
+            metrics: {
+              $setUnion: [
+                {
+                  $ifNull: ["$dependentOn.metrics", []]
+                },
+                {
+                  $reduce: {
+                    input: "$dependentOnMetrics.dependentOn.metrics",
+                    initialValue: [],
+                    in: {
+                      $setUnion: ["$$value", "$$this"]
+                    }
+                  }
+                }
+              ]
+            },
+            measures: {
+              $setUnion: [
+                {
+                  $ifNull: ["$dependentOn.measures", []]
+                },
+                {
+                  $reduce: {
+                    input: "$dependentOnMetrics.dependentOn.measures",
+                    initialValue: [],
+                    in: {
+                      $setUnion: ["$$value", "$$this"]
+                    }
+                  }
+                }
+              ]
+            }
+          }
+        }
+      },
+      {
+        $unset: "dependentOnMetrics"
+      }
+    ]);
+
+    expect(result).toEqual([
+      {
+        _id: "p_commerce_metrics_dtst_v1",
+        category: "dataset",
+        dependentOn: {
+          measures: ["hb_cancel_measure_v1", "hb_booking_measure_v1"],
+          metrics: [
+            "net_booking_count_v1",
+            "cancelled_booking_count_v1",
+            "gross_booking_count_v1"
+          ]
+        }
+      }
+    ]);
+  });
+});