Many-to-many with references in MongoDB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to find all books that a specific author wrote by matching the author's ID.

MongoDB

db.books.find({ authors: [1] })

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

A"authorId789"

B"authorId456"

C"authorId123"

D"authorId000"

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong author ID string.

Not putting the author ID in quotes.

Trying to match the entire authors array instead of an element.

The query filters books where the authors array contains the specific author ID. Replace {{BLANK_1}} with the correct author ID string.

2fill in blank

medium

Complete the code to add a new author ID to a book's authors array using the $push operator.

MongoDB

db.books.updateOne({ _id: bookId }, { $push: { authors: [1] } })

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

AnewAuthorId

B"newAuthorId"

C{ newAuthorId }

D["newAuthorId"]

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting quotes around the author ID.

Using an array or object instead of a string.

Using the wrong update operator.

The $push operator adds a single element to the array. The author ID must be a string, so it needs quotes.

3fill in blank

hard

Fix the error in the aggregation pipeline stage to lookup authors for each book.

MongoDB

{ $lookup: { from: "authors", localField: "authors", foreignField: [1], as: "authorDetails" } }

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

A"_id"

B"id"

C"authorId"

D"authors"

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong field name for foreignField.

Confusing localField and foreignField.

Using a field that does not exist in the authors collection.

The foreignField should be the field in the authors collection that matches the IDs stored in the books' authors array. Usually, this is _id.

4fill in blank

hard

Fill both blanks to create a query that finds authors who wrote a book with a specific title.

MongoDB

db.authors.find({ _id: { $in: db.books.findOne({ title: [1] }).[2] }) })

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

A"The Great Adventure"

Bauthors

CauthorIds

Dwriters

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong field name for author IDs in the book document.

Not quoting the book title string.

Using a field name that does not exist.

The query finds the book by title, then uses the authors array field to find matching author IDs.

5fill in blank

hard

Fill all three blanks to create an aggregation pipeline that joins books with authors and filters books published after 2010.

MongoDB

[ { $match: { year: { $[1]: [2] } } }, { $lookup: { from: "authors", localField: "authors", foreignField: [3], as: "authorInfo" } } ]

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

Agt

B2010

C"_id"

Dgte

Attempts:

3 left

💡 Hint

Common Mistakes

Using $gte instead of $gt when strictly greater is needed.

Not quoting the year number correctly.

Using wrong field names in $lookup.

The $match stage filters books where the year is greater than 2010 using $gt. The $lookup joins authors by matching the _id field.

Practice

1. What does a many-to-many relationship with references mean in MongoDB?

easy

A. Only one collection has references, the other does not.

B. One collection stores all data, the other is empty.

C. Both collections store arrays of IDs pointing to each other.

D. Data is duplicated in both collections without IDs.

Solution

1. Step 1: Understand many-to-many references

   In MongoDB, many-to-many means each document in one collection can link to many documents in another, and vice versa.

2. Step 2: Identify how references are stored

   This is done by storing arrays of IDs in both collections, so each side knows its related documents.

3. Final Answer:

   Both collections store arrays of IDs pointing to each other. -> Option C

4. Quick Check:

   Many-to-many references = arrays of IDs in both collections [OK]

Hint: Many-to-many means both sides hold arrays of IDs [OK]

Common Mistakes:

• Thinking only one side stores references
• Confusing many-to-many with embedding all data
• Assuming data is duplicated without IDs

2. Which of the following is the correct way to store references for a many-to-many relationship between students and courses collections in MongoDB?

easy

A. { _id: 1, name: 'Alice', courses: 101 } and { _id: 101, title: 'Math', students: 1 }

B. { _id: 1, name: 'Alice', courses: [101, 102] } and { _id: 101, title: 'Math', students: [1, 2] }

C. { _id: 1, name: 'Alice', courses: 'Math' } and { _id: 101, title: 'Math', students: 'Alice' }

D. { _id: 1, name: 'Alice', courses: [] } and { _id: 101, title: 'Math', students: [] }

Solution

1. Step 1: Check reference format

   References must be arrays of IDs to represent many-to-many links.

2. Step 2: Validate { _id: 1, name: 'Alice', courses: [101, 102] } and { _id: 101, title: 'Math', students: [1, 2] }

   { _id: 1, name: 'Alice', courses: [101, 102] } and { _id: 101, title: 'Math', students: [1, 2] } shows arrays of IDs in both collections, correctly linking students and courses.

3. Final Answer:

   { _id: 1, name: 'Alice', courses: [101, 102] } and { _id: 101, title: 'Math', students: [1, 2] } -> Option B

4. Quick Check:

   Arrays of IDs in both collections = correct syntax [OK]

Hint: Use arrays of IDs, not single values or names [OK]

Common Mistakes:

• Using single ID instead of array for many-to-many
• Storing names instead of IDs as references
• Leaving reference arrays empty without reason

3. Given these two documents:
{ _id: 1, name: 'Bob', courses: [201, 202] }
{ _id: 201, title: 'History', students: [1, 3] }
What will be the result of querying the courses array for student with _id: 1?

medium

A. ['History']

B. [1, 3]

C. []

D. [201, 202]

Solution

1. Step 1: Identify the student's courses field

   The student document with _id 1 has a courses array: [201, 202].

2. Step 2: Understand the query result

   Querying the courses array for this student returns the IDs [201, 202], not titles or student IDs.

3. Final Answer:

   [201, 202] -> Option D

4. Quick Check:

   Student's courses array = [201, 202] [OK]

Hint: Query returns stored IDs, not linked document fields [OK]

Common Mistakes:

• Confusing course IDs with student IDs
• Expecting course titles instead of IDs
• Assuming empty array if unsure

4. You have these two collections:
students: { _id: 2, name: 'Eve', courses: [301] }
courses: { _id: 301, title: 'Science', students: [] }
What is the problem with this many-to-many reference setup?

medium

A. The students array in courses is missing the student ID.

B. The courses array in students is empty.

C. The student document is missing the courses field.

D. The course document has an invalid _id.

Solution

1. Step 1: Check references in both collections

   The student document lists course 301, but the course document's students array is empty.

2. Step 2: Identify missing reciprocal reference

   For many-to-many, both sides must reference each other. The course's students array should include student ID 2.

3. Final Answer:

   The students array in courses is missing the student ID. -> Option A

4. Quick Check:

   Both sides must reference each other [OK]

Hint: Both sides must list each other's IDs [OK]

Common Mistakes:

• Ignoring missing reciprocal references
• Assuming one side is enough
• Confusing empty arrays with missing fields

5. You want to find all courses taken by students who are enrolled in both course 401 and 402. Given collections with many-to-many references, which MongoDB aggregation pipeline stage helps find students enrolled in both courses?

hard

A. { $match: { courses: { $all: [401, 402] } } }

B. { $match: { courses: { $in: [401, 402] } } }

C. { $match: { courses: 401 } }

D. { $match: { courses: { $size: 2 } } }

Solution

1. Step 1: Understand the query goal

   We want students enrolled in both courses 401 and 402, so their courses array must contain both IDs.

2. Step 2: Choose correct MongoDB operator

   The $all operator matches arrays containing all specified elements, perfect for this case.

3. Final Answer:

   { $match: { courses: { $all: [401, 402] } } } -> Option A

4. Quick Check:

   $all matches arrays containing all elements [OK]

Hint: Use $all to match arrays containing all specified IDs [OK]

Common Mistakes:

• Using $in matches any, not all elements
• Using $size only checks array length, not content
• Matching single course ID misses the second course