$push accumulator for building arrays in MongoDB - Mini Project: Build & Apply

$push accumulator for building arrays in MongoDB

📖 Scenario: You are managing a small online bookstore database. Each book document stores information about the book and a list of reviews from customers. You want to group all reviews by book title to see all feedback in one place.

🎯 Goal: Build an aggregation query using the $push accumulator to collect all reviews for each book title into an array.

📋 What You'll Learn

Create a collection named books with documents containing title and review fields.

Define a variable groupStage for the aggregation $group stage.

Use the $push accumulator inside groupStage to collect reviews into an array for each book title.

Write the complete aggregation pipeline using db.books.aggregate() with the groupStage.

💡 Why This Matters

🌍 Real World

Collecting and grouping user reviews or comments by product or article is common in many applications like e-commerce or blogs.

💼 Career

Understanding aggregation with $push is essential for data analysis and reporting tasks in MongoDB database roles.

1

Create the books collection with sample documents

Create a variable called books and assign it an array with these exact documents: { title: "Book A", review: "Great read" }, { title: "Book B", review: "Informative" }, { title: "Book A", review: "Loved it" }

MongoDB

const books = [
  // Your code here
];

Hint

Use an array of objects with exact title and review fields as shown.

2

Define the groupStage for aggregation

Create a variable called groupStage and assign it an object with a $group key. Group by _id: "$title" and prepare to use $push to collect reviews.

MongoDB

const books = [
  { title: "Book A", review: "Great read" },
  { title: "Book B", review: "Informative" },
  { title: "Book A", review: "Loved it" }
];

const groupStage = {
  // Your code here
};

Hint

Use $group with _id set to "$title" and reviews using $push on "$review".

3

Write the aggregation pipeline using groupStage

Create a variable called pipeline and assign it an array containing the groupStage object.

MongoDB

const books = [
  { title: "Book A", review: "Great read" },
  { title: "Book B", review: "Informative" },
  { title: "Book A", review: "Loved it" }
];

const groupStage = {
  $group: {
    _id: "$title",
    reviews: { $push: "$review" }
  }
};

const pipeline = [
  // Your code here
];

Hint

Put the groupStage object inside an array assigned to pipeline.

4

Complete the aggregation query using db.books.aggregate()

Write a line of code that calls db.books.aggregate(pipeline) and assign the result to a variable called result.

MongoDB

const books = [
  { title: "Book A", review: "Great read" },
  { title: "Book B", review: "Informative" },
  { title: "Book A", review: "Loved it" }
];

const groupStage = {
  $group: {
    _id: "$title",
    reviews: { $push: "$review" }
  }
};

const pipeline = [
  groupStage
];

// Your code here

Hint

Call db.books.aggregate(pipeline) and assign it to result.

Practice

1. What does the $push accumulator do in a MongoDB aggregation pipeline?

easy

A. It removes duplicate values from an array.

B. It adds values to an array for each group in the aggregation.

C. It sorts the documents in the aggregation result.

D. It counts the number of documents in each group.

Solution

1. Step 1: Understand the purpose of $push

   The $push accumulator collects values into an array for each group during aggregation.

2. Step 2: Compare with other accumulators

   Unlike $addToSet which removes duplicates, $push simply appends values, allowing duplicates.

3. Final Answer:

   It adds values to an array for each group in the aggregation. -> Option B

4. Quick Check:

   $push = add values to array [OK]

Hint: Remember: $push always appends values to arrays [OK]

Common Mistakes:

• Confusing $push with $addToSet which removes duplicates
• Thinking $push sorts the array automatically
• Assuming $push counts documents instead of collecting values

2. Which of the following is the correct syntax to use $push in a MongoDB aggregation $group stage?

easy

A. { $group: { _id: "$category", items: { $push: { "$item" } } } }

B. { $group: { _id: "$category", items: { $push: ["$item"] } } }

C. { $group: { _id: "$category", items: { $push: "$item" } } }

D. { $group: { _id: "$category", items: { push: "$item" } } }

Solution

1. Step 1: Review correct $push syntax

   The $push operator requires a field name with a value to push, like $push: "$item".

2. Step 2: Identify syntax errors in options

   { $group: { _id: "$category", items: { $push: ["$item"] } } } incorrectly uses an array around "$item". { $group: { _id: "$category", items: { $push: { "$item" } } } } uses braces instead of quotes. { $group: { _id: "$category", items: { push: "$item" } } } misses the $ sign before push.

3. Final Answer:

   { $group: { _id: "$category", items: { $push: "$item" } } } -> Option C

4. Quick Check:

   Correct $push syntax = { $group: { _id: "$category", items: { $push: "$item" } } } [OK]

Hint: Use $push: "fieldName" inside $group stage [OK]

Common Mistakes:

• Forgetting the $ before push
• Using arrays or objects incorrectly inside $push
• Misplacing quotes around field names

3. Given the collection orders with documents:

{ "customer": "Alice", "product": "Pen" }

{ "customer": "Alice", "product": "Notebook" }

{ "customer": "Bob", "product": "Pen" }

What will be the result of this aggregation?

{ $group: { _id: "$customer", products: { $push: "$product" } } }

medium

A. [{ "_id": "Alice", "products": ["Pen", "Notebook"] }, { "_id": "Bob", "products": ["Pen"] }]

B. [{ "_id": "Alice", "products": "Pen" }, { "_id": "Bob", "products": "Pen" }]

C. [{ "_id": "Alice", "products": ["Notebook"] }, { "_id": "Bob", "products": ["Pen"] }]

D. [{ "_id": "Alice", "products": [] }, { "_id": "Bob", "products": [] }]

Solution

1. Step 1: Group documents by customer

   The aggregation groups documents by the "customer" field, so Alice and Bob form separate groups.

2. Step 2: Use $push to collect products

   For each group, $push collects all "product" values into an array. Alice has "Pen" and "Notebook", Bob has "Pen".

3. Final Answer:

   [{ "_id": "Alice", "products": ["Pen", "Notebook"] }, { "_id": "Bob", "products": ["Pen"] }] -> Option A

4. Quick Check:

   Group by customer + $push products = [{ "_id": "Alice", "products": ["Pen", "Notebook"] }, { "_id": "Bob", "products": ["Pen"] }] [OK]

Hint: Group by field, $push collects all matching values in array [OK]

Common Mistakes:

• Expecting $push to return single value instead of array
• Confusing $push with $first or $last accumulator
• Ignoring grouping effect on output structure

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

{ $group: { _id: "$category", items: { $push: "$item" } } }

What is the likely cause of the error?

medium

A. The field names "category" or "item" do not exist in the documents.

B. You forgot to include the $match stage before $group.

C. The $push operator cannot be used inside $group.

D. The _id field cannot be a string.

Solution

1. Step 1: Check field existence in documents

   If the fields "category" or "item" do not exist in the documents, $push will fail because it tries to access missing fields.

2. Step 2: Validate other options

   $push is valid inside $group, $match is optional, and _id can be a string. So these are not causes of error.

3. Final Answer:

   The field names "category" or "item" do not exist in the documents. -> Option A

4. Quick Check:

   Missing fields cause $push error [OK]

Hint: Verify fields exist before using $push on them [OK]

Common Mistakes:

• Assuming $push requires $match stage
• Thinking _id cannot be string
• Believing $push is invalid inside $group

5. You want to group sales by store and collect all product names sold, but only unique products without duplicates. Which aggregation pipeline correctly uses $push and another operator to achieve this?

hard

A. { $group: { _id: "$store", products: { $push: "$product" } } }

B. { $group: { _id: "$store", products: { $push: { $each: "$product" } } } }

C. { $group: { _id: "$store", products: { $push: { $addToSet: "$product" } } } }

D. { $group: { _id: "$store", products: { $addToSet: "$product" } } }

Solution

1. Step 1: Understand $push vs $addToSet

   $push collects all values including duplicates. $addToSet collects unique values only.

2. Step 2: Identify correct usage for unique array

   To get unique products per store, use $addToSet: "$product" inside $group.

3. Step 3: Analyze other options

   { $group: { _id: "$store", products: { $push: "$product" } } } uses $push (duplicates allowed). { $group: { _id: "$store", products: { $push: { $addToSet: "$product" } } } } nests $addToSet inside $push which is invalid. { $group: { _id: "$store", products: { $push: { $each: "$product" } } } } misuses $each inside $push.

4. Final Answer:

   { $group: { _id: "$store", products: { $addToSet: "$product" } } } -> Option D

5. Quick Check:

   Unique array needs $addToSet, not $push [OK]

Hint: Use $addToSet for unique arrays, $push allows duplicates [OK]

Common Mistakes:

• Using $push when unique values are needed
• Nesting $addToSet inside $push incorrectly
• Misusing $each inside $push accumulator