$push operator for adding to arrays in MongoDB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to add a new tag 'urgent' to the tags array in the document.

MongoDB

db.tasks.updateOne({ _id: 1 }, { [1]: { tags: 'urgent' } })

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

A$addToSet

B$push

C$set

D$inc

Attempts:

3 left

💡 Hint

Common Mistakes

Using $set replaces the whole array instead of adding an element.

Using $inc is for numbers, not arrays.

The $push operator adds a value to an array field. Here, it adds 'urgent' to the tags array.

2fill in blank

medium

Complete the code to add the number 5 to the scores array in all documents where the name is 'Alice'.

MongoDB

db.students.updateMany({ name: 'Alice' }, { [1]: { scores: 5 } })

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

A$set

B$inc

C$push

D$pull

Attempts:

3 left

💡 Hint

Common Mistakes

Using $set replaces the whole array.

Using $pull removes elements, not adds.

$push adds the value 5 to the scores array for all matching documents.

3fill in blank

hard

Fix the error in the code to correctly add 'completed' to the status array for the document with _id 10.

MongoDB

db.orders.updateOne({ _id: 10 }, { [1]: { status: 'completed' } })

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

A$set

B$pull

C$addToSet

D$push

Attempts:

3 left

💡 Hint

Common Mistakes

Using $set replaces the entire status field.

Using $pull removes elements instead of adding.

The $push operator correctly adds 'completed' to the status array. Using $set would replace the field.

4fill in blank

hard

Fill both blanks to add multiple tags 'new' and 'sale' to the tags array in one update.

MongoDB

db.products.updateOne({ _id: 5 }, { [1]: { tags: { [2]: ['new', 'sale'] } } })

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

A$push

B$each

C$addToSet

D$set

Attempts:

3 left

💡 Hint

Common Mistakes

Using $set replaces the whole array.

Not using $each adds the array as a single element.

$push adds elements to an array, and $each allows adding multiple values at once.

5fill in blank

hard

Fill both blanks to add the numbers 10, 20, and 30 to the scores array only if they are not already present.

MongoDB

db.players.updateOne({ name: 'Bob' }, { [1]: { scores: { [2]: [10, 20, 30] } } })

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

A$addToSet

B$push

C$each

D$set

Attempts:

3 left

💡 Hint

Common Mistakes

Using $push adds duplicates.

Not using $each adds the array as one element.

$addToSet adds unique values to an array, and $each allows adding multiple values at once.

Practice

1. What does the $push operator do in MongoDB?

easy

A. Adds a new element to the end of an array field in a document

B. Removes an element from an array field in a document

C. Replaces the entire array field with a new array

D. Creates a new collection in the database

Solution

1. Step 1: Understand the purpose of $push

   The $push operator is used to add elements to an existing array field inside a MongoDB document.

2. Step 2: Compare with other options

   Options A, C, and D describe different operations: removing elements, replacing arrays, or creating collections, which are not what $push does.

3. Final Answer:

   Adds a new element to the end of an array field in a document -> Option A

4. Quick Check:

   $push adds elements [OK]

Hint: Remember: $push always adds to array end [OK]

Common Mistakes:

• Confusing $push with $pull (which removes items)
• Thinking $push replaces the whole array
• Mixing $push with collection creation commands

2. Which of the following is the correct syntax to add the value 5 to the array field numbers in a document where _id is 1?

easy

A. db.collection.updateOne({_id: 1}, {$push: {numbers: 5}})

B. db.collection.updateOne({_id: 1}, {$push: numbers: 5})

C. db.collection.updateOne({_id: 1}, {$push: {numbers => 5}})

D. db.collection.updateOne({_id: 1}, {$push: {numbers: [5]}})

Solution

1. Step 1: Identify correct $push syntax

   The correct syntax uses an object with the field name as key and the value to add as value: {$push: {field: value}}.

2. Step 2: Analyze each option

   db.collection.updateOne({_id: 1}, {$push: {numbers: 5}}) matches the correct syntax. db.collection.updateOne({_id: 1}, {$push: numbers: 5}) misses curly braces around the field-value pair. db.collection.updateOne({_id: 1}, {$push: {numbers => 5}}) uses an invalid arrow syntax. db.collection.updateOne({_id: 1}, {$push: {numbers: [5]}}) pushes the array [5] as a single element instead of adding the scalar value 5.

3. Final Answer:

   db.collection.updateOne({_id: 1}, {$push: {numbers: 5}}) -> Option A

4. Quick Check:

   Correct $push syntax uses {$push: {field: value}} [OK]

Hint: Use {$push: {field: value}} with braces [OK]

Common Mistakes:

• Omitting braces around field-value pair
• Using wrong symbols like => instead of :
• Pushing an array when a single value is intended

3. Given the document { _id: 1, tags: ["red", "blue"] }, what will be the tags array after running db.collection.updateOne({_id: 1}, {$push: {tags: "green"}})?

medium

A. ["red", "blue"]

B. ["red", "green", "blue"]

C. ["green", "red", "blue"]

D. ["red", "blue", "green"]

Solution

1. Step 1: Understand $push effect on array

   The $push operator adds the new element to the end of the existing array.

2. Step 2: Apply $push to the given array

   Starting with ["red", "blue"], pushing "green" adds it at the end, resulting in ["red", "blue", "green"].

3. Final Answer:

   ["red", "blue", "green"] -> Option D

4. Quick Check:

   New item added at array end [OK]

Hint: Remember $push adds at the end of array [OK]

Common Mistakes:

• Assuming $push adds at the start
• Replacing the whole array instead of adding
• Confusing order of elements after push

4. You want to add multiple values ["yellow", "purple"] to the colors array in a document with _id: 2. Which update command will NOT work correctly?

medium

A. db.collection.updateOne({_id: 2}, {$push: {colors: { $each: ["yellow", "purple"] }}})

B. db.collection.updateOne({_id: 2}, {$push: {colors: ["yellow", "purple"]}})

C. db.collection.updateOne({_id: 2}, {$push: {colors: {$each: ["yellow", "purple"]}}})

D. db.collection.updateOne({_id: 2}, {$push: {colors: { $each: ["yellow", "purple" ] }}})

Solution

1. Step 1: Understand how to push multiple items

   To add multiple items to an array, $push must be combined with $each inside the update document.

2. Step 2: Analyze each option

   Options B, C, and D correctly use $each to add multiple values. db.collection.updateOne({_id: 2}, {$push: {colors: ["yellow", "purple"]}}) tries to push an array directly, which will add the entire array as a single element, not multiple elements.

3. Final Answer:

   db.collection.updateOne({_id: 2}, {$push: {colors: ["yellow", "purple"]}}) -> Option B

4. Quick Check:

   Use $each to push multiple items [OK]

Hint: Use $each inside $push for multiple items [OK]

Common Mistakes:

• Pushing an array directly instead of using $each
• Forgetting curly braces around $each
• Using wrong operators like $addToSet instead of $push

5. You have a document { _id: 3, tasks: ["task1"] }. You want to add "task2" and "task3" only if they are not already in the array. Which update command correctly achieves this?

hard

A. db.collection.updateOne({_id: 3}, {$push: {tasks: { $each: ["task2", "task3"] }}})

B. db.collection.updateOne({_id: 3}, {$push: {tasks: { $addToSet: ["task2", "task3"] }}})

C. db.collection.updateOne({_id: 3}, {$addToSet: {tasks: { $each: ["task2", "task3"] }}})

D. db.collection.updateOne({_id: 3}, {$push: {tasks: ["task2", "task3"]}})

Solution

1. Step 1: Understand difference between $push and $addToSet

   $push adds items regardless of duplicates. $addToSet adds items only if they don't exist already.

2. Step 2: Analyze the commands

   db.collection.updateOne({_id: 3}, {$addToSet: {tasks: { $each: ["task2", "task3"] }}}) uses $addToSet with $each to add multiple unique items. Options A and D use $push which can add duplicates. db.collection.updateOne({_id: 3}, {$push: {tasks: { $addToSet: ["task2", "task3"] }}}) incorrectly nests $addToSet inside $push, which is invalid syntax.

3. Final Answer:

   db.collection.updateOne({_id: 3}, {$addToSet: {tasks: { $each: ["task2", "task3"] }}}) -> Option C

4. Quick Check:

   Use $addToSet with $each for unique additions [OK]

Hint: Use $addToSet with $each to avoid duplicates [OK]

Common Mistakes:

• Using $push when duplicates should be avoided
• Nesting $addToSet inside $push incorrectly
• Forgetting $each when adding multiple items