Graph lookup for recursive data in MongoDB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to start a $graphLookup stage in an aggregation pipeline.

MongoDB

{ $graphLookup: { from: "employees", startWith: "$managerId", connectFromField: "[1]", connectToField: "_id", as: "reportingHierarchy" } }

Drag options to blanks, or click blank then click option'

Asubordinates

BemployeeId

CreportsTo

DmanagerId

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong field that does not reference the manager.

Confusing connectFromField with connectToField.

The connectFromField should be the field that holds the reference to the manager, which is managerId.

2fill in blank

medium

Complete the code to specify the field to match in the $graphLookup stage.

MongoDB

{ $graphLookup: { from: "employees", startWith: "$managerId", connectFromField: "managerId", connectToField: "[1]", as: "reportingHierarchy" } }

Drag options to blanks, or click blank then click option'

AmanagerId

B_id

CemployeeId

DreportsTo

Attempts:

3 left

💡 Hint

Common Mistakes

Using a non-unique field for connectToField.

Mixing up connectFromField and connectToField.

The connectToField is usually the unique identifier field, which is _id in MongoDB documents.

3fill in blank

hard

Fix the error in the $graphLookup stage by completing the missing field.

MongoDB

{ $graphLookup: { from: "employees", startWith: "$managerId", connectFromField: "managerId", connectToField: "_id", as: "[1]" } }

Drag options to blanks, or click blank then click option'

AreportingHierarchy

Bsubordinates

Cmanagers

DteamMembers

Attempts:

3 left

💡 Hint

Common Mistakes

Using a vague or unrelated field name.

Leaving the as field empty or incorrect.

The as field defines the name of the array field to store the recursive results. reportingHierarchy clearly describes the data.

4fill in blank

hard

Fill both blanks to complete the $graphLookup stage that limits recursion depth and filters results.

MongoDB

{ $graphLookup: { from: "employees", startWith: "$managerId", connectFromField: "[1]", connectToField: "[2]", as: "team" , maxDepth: 2 } }

Drag options to blanks, or click blank then click option'

AmanagerId

B_id

CemployeeId

DreportsTo

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping the two fields.

Using unrelated field names.

The connectFromField is managerId and connectToField is _id to correctly link employees to their managers.

5fill in blank

hard

Fill all three blanks to create a $graphLookup stage that starts from a specific employee, connects fields correctly, and stores results.

MongoDB

{ $graphLookup: { from: "employees", startWith: "[1]", connectFromField: "[2]", connectToField: "[3]", as: "allReports" } }

Drag options to blanks, or click blank then click option'

A"$employeeId"

BmanagerId

C_id

D"$managerId"

Attempts:

3 left

💡 Hint

Common Mistakes

Not using quotes and $ in startWith.

Mixing up connectFromField and connectToField.

The startWith should be "$managerId" to begin from the manager field, connectFromField is managerId, and connectToField is _id.

Practice

1. What is the primary purpose of the $graphLookup stage in MongoDB aggregation?

easy

A. To create indexes on fields

B. To perform a simple filter on documents

C. To update multiple documents at once

D. To recursively search connected documents within the same collection

Solution

1. Step 1: Understand the role of $graphLookup

   $graphLookup is designed to recursively find related documents by following references within the same collection, useful for hierarchical data.

2. Step 2: Compare with other options

   Options B, C, and D describe other MongoDB operations unrelated to recursive lookup.

3. Final Answer:

   To recursively search connected documents within the same collection -> Option D

4. Quick Check:

   $graphLookup = recursive search [OK]

Hint: Remember $graphLookup is for recursive connected data search [OK]

Common Mistakes:

• Confusing $graphLookup with $match filtering
• Thinking $graphLookup updates documents
• Assuming $graphLookup creates indexes

2. Which of the following is the correct syntax to start a $graphLookup stage in a MongoDB aggregation pipeline?

easy

A. {"$graphLookup": {"from": "employees", "startWith": "$managerId", "connectFromField": "_id", "connectToField": "managerId", "as": "reportingHierarchy"}}

B. {"$graphLookup": {"from": "employees", "startWith": "$managerId", "connectFromField": "managerId", "connectToField": "_id", "as": "reportingHierarchy"}}

C. {"$graphLookup": {"from": "employees", "startWith": "managerId", "connectFromField": "_id", "connectToField": "managerId", "as": "reportingHierarchy"}}

D. {"$graphLookup": {"from": "employees", "startWith": "$id", "connectFromField": "managerId", "connectToField": "_id", "as": "reportingHierarchy"}}

Solution

1. Step 1: Identify correct field references

   In $graphLookup, startWith uses the field to start recursion, usually a reference like "$managerId". The connectToField is the field in the foreign documents to match against the start value, here "_id". The connectFromField is the field in the foreign document used for the next recursion, here "managerId".

2. Step 2: Match syntax with options

   {"$graphLookup": {"from": "employees", "startWith": "$managerId", "connectFromField": "managerId", "connectToField": "_id", "as": "reportingHierarchy"}} correctly uses startWith: "$managerId", connectFromField: "managerId", and connectToField: "_id". Others swap these or use incorrect field names.

3. Final Answer:

   {"$graphLookup": {"from": "employees", "startWith": "$managerId", "connectFromField": "managerId", "connectToField": "_id", "as": "reportingHierarchy"}} -> Option B

4. Quick Check:

   Correct field mapping = {"$graphLookup": {"from": "employees", "startWith": "$managerId", "connectFromField": "managerId", "connectToField": "_id", "as": "reportingHierarchy"}} [OK]

Hint: Match startWith with $field, connectToField with foreign _id [OK]

Common Mistakes:

• Swapping connectFromField and connectToField
• Missing $ in startWith field
• Using wrong field names for connections

3. Given the following collection categories with documents:

{"_id": 1, "name": "Electronics", "parent": null}

{"_id": 2, "name": "Computers", "parent": 1}

{"_id": 3, "name": "Laptops", "parent": 2}

What will be the result of this aggregation?

[ { "$graphLookup": { "from": "categories", "startWith": "$parent", "connectFromField": "parent", "connectToField": "_id", "as": "ancestors" } } ]

for the document with _id: 3?

medium

A. An array 'ancestors' containing documents with _id 2 and 1

B. An empty array 'ancestors'

C. An array 'ancestors' containing only document with _id 1

D. A syntax error due to incorrect $graphLookup usage

Solution

1. Step 1: Understand the recursive lookup

   Starting from the document with _id 3, startWith uses its parent field which is 2. Then it finds documents where _id matches 2, then recursively finds parents of 2, which is 1.

2. Step 2: Trace the ancestors found

   The recursion finds documents with _id 2 and then _id 1, so the 'ancestors' array contains both documents.

3. Final Answer:

   An array 'ancestors' containing documents with _id 2 and 1 -> Option A

4. Quick Check:

   Recursive parents = [2, 1] [OK]

Hint: Trace startWith field value and follow connectToField recursively [OK]

Common Mistakes:

• Assuming only direct parent is returned
• Confusing connectFromField and connectToField
• Expecting syntax error incorrectly

4. Consider this aggregation pipeline snippet:

{ "$graphLookup": { "from": "employees", "startWith": "$managerId", "connectFromField": "managerId", "connectToField": "_id", "as": "team" } }

Why might this pipeline fail or return empty results when run on an employee document with no managerId field?

medium

A. Because as field name is invalid

B. Because connectFromField and connectToField are swapped

C. Because startWith is null or missing, recursion cannot start

D. Because from collection does not exist

Solution

1. Step 1: Check the startWith field value

   If the document has no managerId field, startWith evaluates to null or undefined, so the recursion has no starting point.

2. Step 2: Understand impact on results

   Without a valid start value, $graphLookup returns an empty array for 'team' because it cannot find connected documents.

3. Final Answer:

   Because startWith is null or missing, recursion cannot start -> Option C

4. Quick Check:

   Missing startWith value = empty results [OK]

Hint: Ensure startWith field exists and is not null [OK]

Common Mistakes:

• Thinking connectFromField/connectToField swap causes empty results
• Assuming 'as' field name causes failure
• Ignoring missing 'from' collection errors

5. You have a collection employees where each document has _id and managerId. You want to find all employees under a manager recursively, including the manager. Which $graphLookup configuration correctly achieves this?

hard

A. {"$graphLookup": {"from": "employees", "startWith": "$_id", "connectFromField": "_id", "connectToField": "managerId", "as": "team"}}

B. {"$graphLookup": {"from": "employees", "startWith": "$managerId", "connectFromField": "managerId", "connectToField": "_id", "as": "team"}}

C. {"$graphLookup": {"from": "employees", "startWith": "$_id", "connectFromField": "managerId", "connectToField": "_id", "as": "team"}}

D. {"$graphLookup": {"from": "employees", "startWith": "$managerId", "connectFromField": "_id", "connectToField": "managerId", "as": "team"}}

Solution

1. Step 1: Define the recursion direction

   To find all employees under a manager, start with the manager's _id. Then find employees whose managerId matches this _id, recursively.

2. Step 2: Match fields in $graphLookup

   startWith should be $_id (the manager's id). connectToField is managerId (field in employees to match), and connectFromField is _id (field to match against for next level).

3. Step 3: Verify options

   {"$graphLookup": {"from": "employees", "startWith": "$_id", "connectFromField": "_id", "connectToField": "managerId", "as": "team"}} matches this logic exactly. {"$graphLookup": {"from": "employees", "startWith": "$_id", "connectFromField": "managerId", "connectToField": "_id", "as": "team"}} swaps connectFromField and connectToField incorrectly. The options starting with $managerId are wrong for this use case.

4. Final Answer:

   {"$graphLookup": {"from": "employees", "startWith": "$_id", "connectFromField": "_id", "connectToField": "managerId", "as": "team"}} -> Option A

5. Quick Check:

   Start with manager _id, connectFromField = _id, connectToField = managerId = {"$graphLookup": {"from": "employees", "startWith": "$_id", "connectFromField": "_id", "connectToField": "managerId", "as": "team"}} [OK]

Hint: Start with manager _id, connectFromField = _id, connectToField = managerId [OK]

Common Mistakes:

• Swapping connectFromField and connectToField
• Starting with $managerId instead of $_id
• Confusing direction of recursion