$out and $merge for writing results in MongoDB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to write the aggregation results to a new collection using $out.

MongoDB

db.sales.aggregate([{ $group: { _id: "$item", total: { $sum: "$quantity" } } }, { $[1]: "totalSales" }])

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

Amerge

Bmatch

Cproject

Dout

Attempts:

3 left

💡 Hint

Common Mistakes

Using $merge instead of $out when the goal is to replace the entire collection.

Using $project or $match which do not write results to collections.

The $out stage writes the aggregation results to a specified collection, replacing it if it exists.

2fill in blank

medium

Complete the code to merge aggregation results into an existing collection without replacing it.

MongoDB

db.orders.aggregate([{ $group: { _id: "$customerId", totalSpent: { $sum: "$amount" } } }, { $[1]: { into: "customerTotals", on: "_id", whenMatched: "replace", whenNotMatched: "insert" } }])

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

Aout

Blookup

Cmerge

Dunwind

Attempts:

3 left

💡 Hint

Common Mistakes

Using $out which replaces the entire collection instead of merging.

Using $lookup or $unwind which are unrelated to writing results.

$merge allows merging aggregation results into an existing collection with control over matching and insertion behavior.

3fill in blank

hard

Fix the error in the aggregation pipeline to correctly write results using $merge with the right option key.

MongoDB

db.inventory.aggregate([{ $match: { status: "A" } }, { $merge: { [1]: "activeInventory" } }])

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

Ato

Binto

Ccollection

Dtarget

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'to' or 'collection' which are not valid keys for $merge.

Using 'target' which is not recognized by MongoDB.

The correct option key for specifying the target collection in $merge is 'into'.

4fill in blank

hard

Fill both blanks to write aggregation results to a collection named 'summary' and replace it if it exists.

MongoDB

db.transactions.aggregate([{ $group: { _id: "$type", count: { $sum: 1 } } }, { $[1]: "[2]" }])

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

Aout

Bmerge

Csummary

Dtotals

Attempts:

3 left

💡 Hint

Common Mistakes

Using $merge instead of $out when replacement is desired.

Using wrong collection names.

$out writes results to the specified collection, replacing it. Here, 'summary' is the target collection.

5fill in blank

hard

Fill all three blanks to merge results into 'archive', matching on '_id', and updating existing documents.

MongoDB

db.logs.aggregate([{ $match: { level: "error" } }, { $[1]: { into: "[2]", on: "[3]", whenMatched: "merge", whenNotMatched: "insert" } }])

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

Amerge

Barchive

C_id

Dout

Attempts:

3 left

💡 Hint

Common Mistakes

Using $out instead of $merge for merging.

Using wrong collection or field names.

$merge stage merges documents into 'archive' collection matching on '_id' and merges existing documents.

Practice

1. What is the main difference between the $out and $merge stages in MongoDB aggregation?

easy

A. $out updates documents, and $merge deletes the target collection.

B. $out only works with sharded clusters, $merge does not.

C. $out replaces the entire target collection, while $merge updates or inserts documents based on matching keys.

D. $out appends documents without replacing, $merge replaces the collection.

Solution

1. Step 1: Understand $out behavior

   $out writes aggregation results by replacing the entire target collection with new data.

2. Step 2: Understand $merge behavior

   $merge writes results by updating existing documents or inserting new ones based on matching keys, preserving unmatched documents.

3. Final Answer:

   $out replaces collection; $merge updates/inserts by keys -> Option C

4. Quick Check:

   $out replaces, $merge merges [OK]

Hint: Remember: $out replaces, $merge merges by keys [OK]

Common Mistakes:

• Thinking $out appends data instead of replacing
• Confusing $merge with deleting collections
• Assuming $merge always replaces entire collection

2. Which of the following is the correct syntax to use $out in a MongoDB aggregation pipeline?

easy

A. { $out: "targetCollection" }

B. { $out: { collection: "targetCollection" } }

C. { $out: ["targetCollection"] }

D. { $out: { db: "targetDB" } }

Solution

1. Step 1: Recall $out syntax

   The $out stage requires the target collection name as a string directly, like { $out: "collectionName" }.

2. Step 2: Check options

   { $out: "targetCollection" } matches the correct syntax. The other options use incorrect formats such as extra objects or arrays.

3. Final Answer:

   { $out: "targetCollection" } -> Option A

4. Quick Check:

   $out needs collection name string [OK]

Hint: Use simple string for collection name in $out [OK]

Common Mistakes:

• Using object or array instead of string for collection name
• Trying to specify database inside $out stage
• Adding extra braces or brackets around collection name

3. Given the aggregation pipeline below, what will be the result in the summary collection after running it?

db.sales.aggregate([
  { $group: { _id: "$category", total: { $sum: "$amount" } } },
  { $merge: { into: "summary", on: "_id", whenMatched: "replace", whenNotMatched: "insert" } }
])

medium

A. The summary collection will remain unchanged.

B. The summary collection will be completely replaced with the aggregation results.

C. The aggregation will fail due to missing $out stage.

D. The summary collection will contain documents with total sales per category, replacing existing matching documents.

Solution

1. Step 1: Understand the $group stage

   This groups sales by category and sums amounts, producing documents like { _id: category, total: sum }.

2. Step 2: Understand the $merge stage

   $merge writes results into summary collection, replacing documents with matching _id or inserting new ones.

3. Final Answer:

   summary collection updated with total sales per category, replacing matches -> Option D

4. Quick Check:

   $merge updates or inserts by key [OK]

Hint: Remember $merge updates matching keys, inserts new [OK]

Common Mistakes:

• Thinking $merge replaces entire collection like $out
• Assuming aggregation fails without $out
• Believing collection stays unchanged after $merge

4. You run this aggregation pipeline but get an error:

db.orders.aggregate([
  { $match: { status: "shipped" } },
  { $out: { collection: "shippedOrders" } }
])

What is the likely cause of the error?

medium

A. You cannot use $out after $match stage.

B. The $out stage syntax is incorrect; it should be a string, not an object.

C. The $match stage is invalid and causes the error.

D. The target collection shippedOrders does not exist.

Solution

1. Step 1: Check $out syntax

   $out requires the target collection name as a string, not an object with a collection field.

2. Step 2: Analyze error cause

   Using { collection: "shippedOrders" } causes syntax error; correct usage is { $out: "shippedOrders" }.

3. Final Answer:

   Incorrect $out syntax using object instead of string -> Option B

4. Quick Check:

   $out needs string collection name [OK]

Hint: Use string for $out target, not object [OK]

Common Mistakes:

• Assuming $match causes error
• Thinking $out requires existing collection
• Believing $out cannot follow $match

5. You want to aggregate user activity data and save results into a collection named userStats. You want to update existing documents by userId and insert new ones if missing, but keep other fields unchanged when matched. Which $merge option should you use?

hard

A. { into: "userStats", on: "userId", whenMatched: "merge", whenNotMatched: "insert" }

B. { into: "userStats", on: "userId", whenMatched: "replace", whenNotMatched: "insert" }

C. { into: "userStats", on: "userId", whenMatched: "fail", whenNotMatched: "insert" }

D. { into: "userStats", on: "userId", whenMatched: "keepExisting", whenNotMatched: "discard" }

Solution

1. Step 1: Understand whenMatched options

   whenMatched: "merge" updates only specified fields, keeping others unchanged; replace overwrites entire document.

2. Step 2: Choose correct $merge options

   To update existing documents by userId and insert new ones, use whenMatched: "merge" and whenNotMatched: "insert".

3. Final Answer:

   { into: "userStats", on: "userId", whenMatched: "merge", whenNotMatched: "insert" } -> Option A

4. Quick Check:

   whenMatched: "merge" updates fields, inserts new [OK]

Hint: Use whenMatched: "merge" to update fields, not replace [OK]

Common Mistakes:

• Using replace which overwrites entire documents
• Choosing fail which errors on matches
• Using invalid whenNotMatched options