One-to-many referencing pattern in MongoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the one-to-many referencing pattern in MongoDB?

It is a way to link one document to many other documents by storing references (usually ObjectIds) in the related documents instead of embedding all data in one place.

beginner

Why use referencing instead of embedding in a one-to-many relationship?

Referencing keeps documents smaller and avoids duplication. It is useful when the 'many' side can grow large or changes often, so you don't store all data inside one document.

beginner

How do you store references in MongoDB for one-to-many relationships?

You store the ObjectId of the 'one' document inside the 'many' documents as a field, like 'authorId' in posts pointing to an author document.

intermediate

What MongoDB operation helps to get related documents using references?

The $lookup aggregation stage lets you join documents from two collections using references, similar to a SQL join.

beginner

Give an example of a one-to-many referencing pattern in MongoDB collections.

A 'users' collection with user documents, and a 'posts' collection where each post has a 'userId' field referencing the user who wrote it.

In MongoDB, where do you store the reference in a one-to-many relationship?

A In a separate collection only

B In the 'many' side documents

C In the 'one' side document

D You embed all data in one document

The reference is stored in the 'many' side documents to point back to the 'one' side document.

Which MongoDB feature allows combining data from two collections using references?

A $match

B $project

C $group

D $lookup

$lookup is used to join documents from different collections based on references.

Why might you choose referencing over embedding in MongoDB?

A Because referencing is faster for all queries

B To store all data in one place

C To keep documents small and avoid duplication

D Because embedding is not supported

Referencing helps keep documents smaller and avoids duplicating data when the 'many' side is large or changes often.

What type of value is typically used as a reference in MongoDB?

A ObjectId

B String

C Integer

D Boolean

ObjectId is the standard unique identifier used as a reference between documents.

In a blog app, where would you store the author reference for each post?

A Inside each post document as 'authorId'

B Inside the author document as a list of posts

C In a separate collection unrelated to posts

D You embed the entire author document inside each post

Each post stores the 'authorId' to reference the author document, following the one-to-many referencing pattern.

Explain the one-to-many referencing pattern in MongoDB and why it is useful.

Think about how to link one document to many others without copying all data.

Describe how you would retrieve related documents in MongoDB using the one-to-many referencing pattern.

Consider how to combine data from two collections based on stored references.

Practice

1. What is the main purpose of the one-to-many referencing pattern in MongoDB?

easy

A. To create a backup of documents automatically

B. To store all related data inside a single document

C. To link one document to many related documents using their IDs

D. To encrypt documents for security

Solution

1. Step 1: Understand one-to-many referencing

   It connects one document to many others by storing their IDs, not embedding all data.

2. Step 2: Compare options

   Only To link one document to many related documents using their IDs describes linking documents by IDs, which is the core of one-to-many referencing.

3. Final Answer:

   To link one document to many related documents using their IDs -> Option C

4. Quick Check:

   One-to-many referencing = linking documents by IDs [OK]

Hint: One-to-many means one document points to many IDs [OK]

Common Mistakes:

• Thinking all data is stored inside one document
• Confusing referencing with embedding
• Assuming it creates backups automatically

2. Which of the following is the correct way to reference many comments in a post document using one-to-many referencing in MongoDB?

easy

A. { _id: 1, title: 'Post', comments: [ObjectId('abc123'), ObjectId('def456')] }

B. { _id: 1, title: 'Post', comments: { text: 'Nice post!' } }

C. { _id: 1, title: 'Post', comments: 'This is a comment' }

D. { _id: 1, title: 'Post', comments: ObjectId('abc123') }

Solution

1. Step 1: Identify referencing format

   One-to-many referencing stores an array of ObjectIds pointing to related documents.

2. Step 2: Check each option

   { _id: 1, title: 'Post', comments: [ObjectId('abc123'), ObjectId('def456')] } uses an array of ObjectIds, which is correct. Others embed data or use a single ID incorrectly.

3. Final Answer:

   { _id: 1, title: 'Post', comments: [ObjectId('abc123'), ObjectId('def456')] } -> Option A

4. Quick Check:

   Array of ObjectIds = correct referencing [OK]

Hint: Use array of ObjectIds to reference many documents [OK]

Common Mistakes:

• Embedding comment text instead of IDs
• Using a single ObjectId for many comments
• Storing comments as plain strings

3. Given these two collections:
posts: [{ _id: 1, title: 'Hello', comments: [101, 102] }]
comments: [{ _id: 101, text: 'Nice!' }, { _id: 102, text: 'Great!' }]
What will be the result of this query to find comments for post with _id 1?
db.comments.find({ _id: { $in: db.posts.findOne({ _id: 1 }).comments } })

medium

A. [] (empty array)

B. [{ _id: 1, title: 'Hello', comments: [101, 102] }]

C. SyntaxError due to nested query

D. [{ _id: 101, text: 'Nice!' }, { _id: 102, text: 'Great!' }]

Solution

1. Step 1: Understand the query

   The inner findOne gets the comments array [101, 102] from post _id 1.

2. Step 2: Use $in to find matching comments

   The outer find returns comments with _id in [101, 102], so both comment documents are returned.

3. Final Answer:

   [{ _id: 101, text: 'Nice!' }, { _id: 102, text: 'Great!' }] -> Option D

4. Quick Check:

   Find comments by IDs in post = matching comment docs [OK]

Hint: Use $in with IDs array to get related documents [OK]

Common Mistakes:

• Expecting post document instead of comments
• Thinking nested query causes syntax error
• Getting empty result by mistake

4. You have this post document:
{ _id: 1, title: 'Test', comments: ['abc', 'def'] }
But when you run db.comments.find({ _id: { $in: post.comments } }), it returns no results. What is the most likely problem?

medium

A. ObjectIds in post.comments are strings, not ObjectId type

B. The comments collection has no documents with those ObjectIds

C. The query syntax is incorrect for $in operator

D. The post document is missing the comments field

Solution

1. Step 1: Check data types of referenced IDs

   If post.comments contains strings instead of ObjectId objects, the query won't match.

2. Step 2: Confirm query correctness

   The query syntax is correct, so the mismatch is likely due to type difference.

3. Final Answer:

   ObjectIds in post.comments are strings, not ObjectId type -> Option A

4. Quick Check:

   Type mismatch in IDs causes no matches [OK]

Hint: Ensure referenced IDs are ObjectId type, not strings [OK]

Common Mistakes:

• Assuming empty results mean syntax error
• Ignoring data type differences
• Thinking missing field causes this error

5. You want to design a blog system with users, posts, and comments. Each user can write many posts, and each post can have many comments. Using one-to-many referencing, which is the best way to organize references?

hard

A. comments store userId; posts store arrays of commentIds

B. posts store userId; comments store postId; users do not store references

C. users embed all posts; posts embed all comments

D. users store arrays of postIds; posts store arrays of commentIds

Solution

1. Step 1: Understand one-to-many referencing best practice

   Usually, the 'many' side stores the reference to the 'one' side to avoid large arrays.

2. Step 2: Apply to blog system

   Posts store userId (one user, many posts), comments store postId (one post, many comments), users don't store arrays.

3. Final Answer:

   posts store userId; comments store postId; users do not store references -> Option B

4. Quick Check:

   Many side stores reference to one side [OK]

Hint: Store reference on 'many' side to 'one' side document [OK]

Common Mistakes:

• Embedding all posts or comments inside users or posts
• Storing large arrays on 'one' side documents
• Confusing which side stores the reference