$push operator for adding to arrays in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the result of this $push update?

Given a document { _id: 1, fruits: ["apple", "banana"] }, what will be the fruits array after running this update?

db.collection.updateOne({ _id: 1 }, { $push: { fruits: "orange" } })

A ["apple", "banana", "orange"]

B ["orange"]

C ["apple", "banana"]

D ["apple", "banana", ["orange"]]

💡 Hint

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

The $push operator appends the specified value to the array field. Here, "orange" is added to the end of the existing fruits array.

📝 Syntax

intermediate

Which update query correctly uses $push to add multiple elements?

You want to add the elements "grape" and "melon" to the fruits array in one update. Which query is correct?

A db.collection.updateOne({ _id: 1 }, { $push: { fruits: { $each: ["grape", "melon"] } } })

B db.collection.updateOne({ _id: 1 }, { $push: { fruits: { $append: ["grape", "melon"] } } })

C db.collection.updateOne({ _id: 1 }, { $push: { fruits: ["grape", "melon"] } })

D db.collection.updateOne({ _id: 1 }, { $push: { fruits: { $add: ["grape", "melon"] } } })

💡 Hint

Use $each inside $push to add multiple elements.

To add multiple elements with $push, you must use the $each modifier. Options B and C use invalid operators, and A pushes the entire array as one element.

❓ optimization

advanced

How to efficiently add unique elements to an array?

You want to add "kiwi" to the fruits array only if it does not already exist. Which update query achieves this efficiently?

A db.collection.updateOne({ _id: 1 }, { $push: { fruits: "kiwi" } })

B db.collection.updateOne({ _id: 1 }, { $addToSet: { fruits: "kiwi" } })

C db.collection.updateOne({ _id: 1, fruits: { $ne: "kiwi" } }, { $push: { fruits: "kiwi" } })

D db.collection.updateOne({ _id: 1 }, { $push: { fruits: { $unique: "kiwi" } } })

💡 Hint

Use an operator designed to add unique elements to arrays.

$addToSet adds the element only if it is not already present. $push always adds the element, possibly creating duplicates.

🧠 Conceptual

advanced

What happens if you $push to a non-array field?

Consider a document { _id: 1, fruits: "apple" }. What happens if you run:

db.collection.updateOne({ _id: 1 }, { $push: { fruits: "banana" } })

A The update ignores the $push and leaves the field unchanged

B The update converts the field to an array and adds "banana"

C The update fails with an error because the field is not an array

D The field is replaced with the string "banana"

💡 Hint

Think about the data type requirements for $push.

$push requires the target field to be an array. If it is not, the update fails with an error.

🔧 Debug

expert

Why does this $push update not add the element?

You run this update:

db.collection.updateOne({ _id: 1 }, { $push: { fruits: { $each: ["pear"], $slice: 2 } } })

The fruits array was ["apple", "banana"] before. After the update, it remains unchanged. Why?

A Because $each requires $sort to work properly with $slice

B Because $slice must be negative to keep the last elements, so the update removed all elements

C Because $slice is not a valid modifier inside $push

D Because $slice with a positive number limits the array to that many elements from the start, but the update did not add new elements first

💡 Hint

Understand how $slice works with $push and $each.

$slice limits the array size after adding new elements. A positive $slice keeps only the first N elements. Since the array was already size 2, adding one element and slicing to 2 keeps only the first two elements, effectively discarding the new one.

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