$replaceRoot for restructuring in MongoDB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to replace the root document with the "details" field.

MongoDB

db.collection.aggregate([{ $replaceRoot: { newRoot: [1] } }])

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

A$$ROOT

B$details

C$root

D$_id

Attempts:

3 left

💡 Hint

Common Mistakes

Using $$ROOT instead of $details causes no change.

Using $root or $_id replaces with wrong fields.

The $replaceRoot stage replaces the root document with the specified field. Using $details sets the root to the "details" field.

2fill in blank

medium

Complete the code to replace the root with the "info" field inside "profile".

MongoDB

db.collection.aggregate([{ $replaceRoot: { newRoot: [1] } }])

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

A$info.profile

B$profile.info.details

C$profile_info

D$profile.info

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping the order of fields like $info.profile is incorrect.

Using underscores instead of dots breaks the path.

To replace the root with a nested field, use the full path with dot notation. Here, $profile.info points to the "info" field inside "profile".

3fill in blank

hard

Fix the error in the code to correctly replace the root with the "address" field.

MongoDB

db.collection.aggregate([{ $replaceRoot: { newRoot: [1] } }])

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

A$address

Baddress

C$$address

Daddress$

Attempts:

3 left

💡 Hint

Common Mistakes

Omitting the dollar sign causes an error.

Using double dollar signs or trailing dollar signs is invalid.

The newRoot value must be a field path starting with a dollar sign. Using $address correctly references the "address" field.

4fill in blank

hard

Fill both blanks to replace the root with the "contact" field and rename it to "info".

MongoDB

db.collection.aggregate([{ $replaceRoot: { newRoot: { [1]: [2] } } }])

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

Ainfo

B$contact

C$info

Dcontact

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong field path for the value.

Swapping key and value positions.

To rename the root field, create a new object with the desired key "info" and assign it the value of the "contact" field using $contact.

5fill in blank

hard

Fill all three blanks to replace the root with a new object containing "name", "email", and "phone" fields from "user".

MongoDB

db.collection.aggregate([{ $replaceRoot: { newRoot: { name: [1], email: [2], phone: [3] } } }])

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

A$user.name

B$user.email

C$user.phone

D$user.contact

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong field paths or mixing keys.

Using a non-existent field like $user.contact.

Each field in the new root object should be assigned the corresponding nested field from "user" using dot notation with a dollar sign.

Practice

1. What does the $replaceRoot stage do in a MongoDB aggregation pipeline?

easy

A. It groups documents by a specified key.

B. It filters documents based on a condition.

C. It replaces the entire document with the specified embedded document.

D. It sorts documents by a specified field.

Solution

1. Step 1: Understand the purpose of $replaceRoot

   $replaceRoot replaces the whole document with a specified nested document inside it.

2. Step 2: Compare with other stages

   Filtering, sorting, and grouping are done by $match, $sort, and $group respectively, not $replaceRoot.

3. Final Answer:

   It replaces the entire document with the specified embedded document. -> Option C

4. Quick Check:

   $replaceRoot changes document root [OK]

Hint: Remember: $replaceRoot swaps the whole document root [OK]

Common Mistakes:

• Confusing $replaceRoot with filtering or sorting stages
• Thinking it only modifies part of the document
• Assuming it adds fields instead of replacing

2. Which of the following is the correct syntax to use $replaceRoot to replace the root with the details field?

easy

A. { $replaceRoot: { newRoot: "$details" } }

B. { $replaceRoot: { root: "$details" } }

C. { $replaceRoot: { newRoot: details } }

D. { $replaceRoot: { newRoot: "$details" }, $match: {} }

Solution

1. Step 1: Recall correct $replaceRoot syntax

   The correct syntax requires newRoot with a string path starting with "$" to specify the field.

2. Step 2: Analyze options

   { $replaceRoot: { newRoot: "$details" } } uses newRoot: "$details" which is correct. { $replaceRoot: { root: "$details" } } uses wrong key root. { $replaceRoot: { newRoot: details } } misses quotes around field path. { $replaceRoot: { newRoot: "$details" }, $match: {} } adds an unrelated $match inside the same stage, which is invalid.

3. Final Answer:

   { $replaceRoot: { newRoot: "$details" } } -> Option A

4. Quick Check:

   Use newRoot: "$field" syntax [OK]

Hint: Use newRoot: "$fieldName" exactly [OK]

Common Mistakes:

• Omitting quotes around field path
• Using wrong key like 'root' instead of 'newRoot'
• Combining stages incorrectly

3. Given the collection documents:

{ "name": "Alice", "info": { "age": 30, "city": "NY" } }

What will be the output of this aggregation?

[{ $replaceRoot: { newRoot: "$info" } }]

medium

A. [{ "name": "Alice", "info": { "age": 30, "city": "NY" } }]

B. [{ "age": 30, "city": "NY" }]

C. [{ "info": { "age": 30, "city": "NY" } }]

D. [{ "age": 30 }]

Solution

1. Step 1: Understand $replaceRoot effect

   The stage replaces the whole document with the content of the info field.

2. Step 2: Apply to given document

   Original document has info as { "age": 30, "city": "NY" }, so output document is exactly that object.

3. Final Answer:

   [{ "age": 30, "city": "NY" }] -> Option B

4. Quick Check:

   Root replaced by info field content [OK]

Hint: Output matches the nested field content exactly [OK]

Common Mistakes:

• Expecting original fields to remain
• Confusing nested field with root
• Selecting wrong output format

4. You wrote this aggregation but it throws an error:

[{ $replaceRoot: { newRoot: info } }]

What is the error and how to fix it?

medium

A. Missing quotes around field path; fix with newRoot: "$info".

B. Wrong operator; use $setRoot instead of $replaceRoot.

C. Field info does not exist; add it first.

D. Aggregation pipeline must be an object, not an array.

Solution

1. Step 1: Identify syntax error in newRoot value

   The field path must be a string starting with "$" to indicate a field reference.

2. Step 2: Correct the syntax

   Change newRoot: info to newRoot: "$info" to fix the error.

3. Final Answer:

   Missing quotes around field path; fix with newRoot: "$info". -> Option A

4. Quick Check:

   Field paths need quotes and $ prefix [OK]

Hint: Always quote field paths with $ prefix [OK]

Common Mistakes:

• Omitting quotes around field names
• Confusing $replaceRoot with $setRoot
• Misunderstanding pipeline array structure

5. You have documents:

{ "user": { "id": 1, "name": "Bob" }, "score": 50 }

How do you write an aggregation pipeline using $replaceRoot to make the output documents only contain the user fields plus the score field at the root level?

hard

A. [{ $replaceRoot: { newRoot: "$score" } }]

B. [{ $replaceRoot: { newRoot: "$user" } }, { $addFields: { score: "$score" } }]

C. [{ $project: { user: 1, score: 1 } }]

D. [{ $replaceRoot: { newRoot: { $mergeObjects: ["$user", { score: "$score" }] } } }]

Solution

1. Step 1: Understand the goal

   We want to flatten user fields to root and keep score alongside.

2. Step 2: Use $mergeObjects inside $replaceRoot

   $mergeObjects combines user object and { score: "$score" } into one root document.

3. Step 3: Check options

   [{ $replaceRoot: { newRoot: { $mergeObjects: ["$user", { score: "$score" }] } } }] correctly merges and replaces root. [{ $replaceRoot: { newRoot: "$user" } }, { $addFields: { score: "$score" } }] tries to add fields after replacing root, but score is lost after first stage. [{ $project: { user: 1, score: 1 } }] only projects fields, not restructuring. [{ $replaceRoot: { newRoot: "$score" } }] replaces root with score only, losing user.

4. Final Answer:

   [{ $replaceRoot: { newRoot: { $mergeObjects: ["$user", { score: "$score" }] } } }] -> Option D

5. Quick Check:

   Use $mergeObjects to combine fields [OK]

Hint: Combine objects with $mergeObjects inside $replaceRoot [OK]

Common Mistakes:

• Replacing root with only one field losing others
• Trying to add fields after $replaceRoot without preserving data
• Using $project instead of restructuring