$each modifier with $push in MongoDB - Mini Project: Build & Apply

$each modifier with $push in MongoDB

📖 Scenario: You are managing a MongoDB collection that stores information about books in a library. Each book document has a title and an array called genres that lists the genres the book belongs to.Sometimes, new genres need to be added to a book's genres array all at once.

🎯 Goal: Learn how to use the $push operator with the $each modifier to add multiple genres to the genres array of a book document in a single update operation.

📋 What You'll Learn

Create a collection called books with one book document.

Add a variable to hold the new genres to add.

Use $push with $each to add multiple genres to the genres array.

Update the book document to include the new genres.

💡 Why This Matters

🌍 Real World

In real-world applications, you often need to update array fields in database documents by adding multiple items at once, such as tags, categories, or genres.

💼 Career

Knowing how to efficiently update arrays in MongoDB using $push with $each is a common task for backend developers and database administrators working with NoSQL databases.

1

Create the initial books collection with one book document

Create a collection called books and insert one document with title set to 'The Great Adventure' and genres set to an array containing 'Adventure' and 'Fantasy'.

MongoDB

// Insert one document into the books collection
// Your code here

Hint

Use db.books.insertOne() with a document containing title and genres fields.

2

Create a variable with new genres to add

Create a variable called newGenres and set it to an array containing 'Mystery' and 'Thriller'.

MongoDB

db.books.insertOne({ title: 'The Great Adventure', genres: ['Adventure', 'Fantasy'] })
// Create a variable called newGenres with ['Mystery', 'Thriller']
// Your code here

Hint

Use const newGenres = ['Mystery', 'Thriller'] to create the array.

3

Use $push with $each to add multiple genres

Write an update query that uses db.books.updateOne() to find the document with title equal to 'The Great Adventure' and uses $push with $each to add all genres from the newGenres array to the genres array.

MongoDB

db.books.insertOne({ title: 'The Great Adventure', genres: ['Adventure', 'Fantasy'] })
const newGenres = ['Mystery', 'Thriller']
// Use updateOne with $push and $each to add newGenres to genres array
// Your code here

Hint

Use db.books.updateOne() with a filter on title and an update using { $push: { genres: { $each: newGenres } } }.

4

Verify the update by querying the updated document

Write a query using db.books.findOne() to find the document with title equal to 'The Great Adventure' to confirm the genres array now includes 'Mystery' and 'Thriller'.

MongoDB

db.books.insertOne({ title: 'The Great Adventure', genres: ['Adventure', 'Fantasy'] })
const newGenres = ['Mystery', 'Thriller']
db.books.updateOne(
  { title: 'The Great Adventure' },
  { $push: { genres: { $each: newGenres } } }
)
// Query the updated document with findOne
// Your code here

Hint

Use db.books.findOne({ title: 'The Great Adventure' }) to see the updated document.

Practice

1. What does the $each modifier do when used with $push in MongoDB?

easy

A. It sorts the array elements after pushing a new value.

B. It adds multiple values to an array in a single update operation.

C. It replaces the entire array with a new set of values.

D. It removes multiple values from an array.

Solution

1. Step 1: Understand $push usage

   $push adds elements to an array field in a document.

2. Step 2: Role of $each modifier

   $each allows adding multiple elements at once inside $push, making the update efficient.

3. Final Answer:

   It adds multiple values to an array in a single update operation. -> Option B

4. Quick Check:

   $each adds many items [OK]

Hint: Remember: $each adds many items inside $push [OK]

Common Mistakes:

• Thinking $each removes items
• Using $each outside $push
• Confusing $each with sorting

2. Which of the following is the correct syntax to add multiple values ["a", "b", "c"] to the array field tags using $push with $each?

easy

A. { $push: { tags: { $each: ["a", "b", "c"] } } }

B. { $push: { tags: ["a", "b", "c"] } }

C. { $push: { tags: { $add: ["a", "b", "c"] } } }

D. { $push: { tags: { $each: "a", "b", "c" } } }

Solution

1. Step 1: Check correct $push syntax

   $push requires a field and a value or modifier object.

2. Step 2: Validate $each usage

   $each must be an array inside $push to add multiple items.

3. Final Answer:

   { $push: { tags: { $each: ["a", "b", "c"] } } } -> Option A

4. Quick Check:

   Correct $each syntax = { $push: { tags: { $each: ["a", "b", "c"] } } } [OK]

Hint: Use $each with an array inside $push [OK]

Common Mistakes:

• Passing array directly without $each
• Using invalid operator like $add
• Incorrect $each format without array brackets

3. Given the document { _id: 1, colors: ["red"] }, what will be the colors array after running this update?

{ $push: { colors: { $each: ["blue", "green"] } } }

medium

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

B. ["blue", "green"]

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

D. ["red"]

Solution

1. Step 1: Initial array and update operation

   Original array is ["red"]. The update uses $push with $each to add "blue" and "green".

2. Step 2: Effect of $each inside $push

   $each adds each element individually, so "blue" and "green" are appended to the array.

3. Final Answer:

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

4. Quick Check:

   Array after update = ["red", "blue", "green"] [OK]

Hint: $each adds items individually, not as a nested array [OK]

Common Mistakes:

• Expecting nested array inside colors
• Replacing array instead of pushing
• Ignoring original array elements

4. What is wrong with this update operation?

{ $push: { items: { $each: "apple", "banana" } } }

medium

A. The $each value must be an array, not separate strings.

B. The $push operator cannot be used with $each.

C. The field name items is invalid.

D. The update syntax is correct and will work as expected.

Solution

1. Step 1: Analyze $each value type

   $each requires an array of values, but here it is given as separate strings without array brackets.

2. Step 2: Identify syntax error

   Passing multiple strings without array brackets causes a syntax error in MongoDB update.

3. Final Answer:

   The $each value must be an array, not separate strings. -> Option A

4. Quick Check:

   $each needs array brackets [OK]

Hint: Always wrap $each values in square brackets [OK]

Common Mistakes:

• Passing multiple values without array brackets
• Confusing $each with $addToSet
• Assuming $push alone can add multiple items

5. You want to add multiple new tags ["urgent", "review", "backend"] to the tags array of a document, but only if they are not already present. Which update operation correctly uses $push with $each and ensures no duplicates?

hard

A. { $push: { tags: { $each: ["urgent", "review", "backend"] } } }

B. { $addToSet: { tags: ["urgent", "review", "backend"] } }

C. { $push: { tags: ["urgent", "review", "backend"] } }

D. { $addToSet: { tags: { $each: ["urgent", "review", "backend"] } } }

Solution

1. Step 1: Understand requirement to avoid duplicates

   $push adds items regardless of duplicates, so it doesn't fit the requirement.

2. Step 2: Use $addToSet with $each

   $addToSet adds unique values only, and supports $each to add multiple items without duplicates.

3. Final Answer:

   { $addToSet: { tags: { $each: ["urgent", "review", "backend"] } } } -> Option D

4. Quick Check:

   $addToSet + $each avoids duplicates [OK]

Hint: Use $addToSet with $each to add unique items [OK]

Common Mistakes:

• Using $push which allows duplicates
• Not using $each with $addToSet
• Passing array directly without $each