Graph lookup for recursive data in MongoDB - Step-by-Step Execution

Concept Flow - Graph lookup for recursive data

Start with root node

↓

Lookup connected nodes

↓

Add connected nodes to results

↓

For each connected node

↓

Repeat lookup recursively

↓

Stop when no new nodes found

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Return full recursive graph

Start from a root node, find connected nodes recursively, and collect all related nodes until no new nodes are found.

Execution Sample

MongoDB

db.nodes.aggregate([
  {
    $graphLookup: {
      from: "nodes",
      startWith: "$_id",
      connectFromField: "parent",
      connectToField: "_id",
      as: "ancestors"
    }
  }
])

This query recursively finds all ancestor nodes starting from the '_id' field of each document.

Execution Table

Step	Action	Current Node _id	Connected Nodes Found	Accumulated Results
1	Start with root node	5	parent: 3	[5]
2	Lookup connected nodes	3	parent: 1	[5, 3]
3	Lookup connected nodes	1	parent: null	[5, 3, 1]
4	No more connected nodes	null	none	[5, 3, 1]
5	Return full recursive graph	null	none	[5, 3, 1]

💡 No more connected nodes found, recursion ends.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
Current Node _id	null	5	3	1	null
Connected Nodes Found	[]	[3]	[1]	[]	[]
Accumulated Results	[]	[5]	[5, 3]	[5, 3, 1]	[5, 3, 1]

Key Moments - 2 Insights

Why does the recursion stop when the connected node is null?

Because in the execution_table at Step 3 and 4, no new connected nodes are found (parent is null), so the recursion ends.

How does $graphLookup avoid infinite loops in recursive data?

It tracks accumulated results (see Accumulated Results column) and stops when no new nodes are found, preventing revisiting the same nodes.

Visual Quiz - 3 Questions

Look at the execution_table, what is the Accumulated Results after Step 2?

A [3]

B [5, 3]

C [5]

D [5, 3, 1]

💡 Hint

Check the 'Accumulated Results' column in the execution_table row for Step 2.

At which step does the recursion find no more connected nodes?

A Step 2

B Step 3

C Step 4

D Step 1

💡 Hint

Look at the 'Connected Nodes Found' column in the execution_table to find where it becomes 'none'.

If the root node had no parent, what would be the Accumulated Results after Step 1?

A [root node id]

B [parent id]

C []

D null

💡 Hint

Refer to variable_tracker for how Accumulated Results starts with the root node id.

Concept Snapshot

Use $graphLookup to recursively find connected documents.
Start with a field (startWith).
Connect from one field to another (connectFromField, connectToField).
Collect all connected nodes in an array (as).
Stops when no new nodes are found to avoid infinite loops.

Full Transcript

This visual execution shows how MongoDB's $graphLookup works to find recursive data. Starting from a root node, it looks up connected nodes by matching fields. Each connected node is added to the results. The process repeats recursively for each connected node until no new nodes are found. The accumulated results grow step by step, and recursion stops when no more connections exist. This prevents infinite loops and collects the full recursive graph data.