- A
Move the friends list to a subcollection under each user document.
Why wrong: Subcollections are good for one-to-many, but for many-to-many relationships, a separate collection is often better.
- B
Migrate user profiles and friendships to Cloud SQL for relational capabilities.
Why wrong: This is a complete database migration, not a schema redesign within Firestore.
- C
Limit the maximum size of the friends array to 1000 at the application level.
Why wrong: This is an application constraint, not a scalable schema design.
- D
Create a new 'friendships' collection with documents containing user_id_1 and user_id_2 fields.
A separate collection for relationships scales well and avoids large documents.
Question 499 of 1,000
Designing Many-to-Many Relationships in Firestore: Friends List Example
This PCDE practice question tests your understanding of design and implement database schemas. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A mobile app backend uses Firestore for user profiles. The schema has a single collection 'users' where each document contains: user_id (used as document ID), name, email, and friends (an array of user IDs). The friends array can grow large (thousands of IDs). When a user adds a friend, the application updates the array, causing the document to grow and leading to write contention and size limit warnings. The team needs to redesign the schema to scale better. What is the best approach?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue:
"best"Why it matters: Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
Quick Answer
The correct approach is to create a new 'friendships' collection with documents containing user_id_1 and user_id_2 fields. This redesign solves the scaling problem by moving from a single document with an ever-growing array to a separate collection where each document represents a single friendship edge, eliminating write contention and the 1 MiB document size limit. On the Google Professional Cloud Database Engineer exam, this scenario tests your understanding of Firestore's data modeling best practices for many-to-many relationships, specifically the trade-off between denormalized arrays and normalized edge collections—a common trap is assuming subcollections work for many-to-many when they actually model one-to-many hierarchies. A useful memory tip: think of a social graph where each friendship is a separate node in a dedicated collection, not a list crammed into a user’s profile.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Create a new 'friendships' collection with documents containing user_id_1 and user_id_2 fields.
Option D is correct because it normalizes the friendship relationship into a separate 'friendships' collection, where each document represents a single bidirectional link between two users. This avoids unbounded document growth and write contention on user documents, as adding a friend only requires a small write to a new friendship document rather than updating a potentially large array. Firestore's 1 MiB document size limit and 1 write per second per document limit are no longer risk factors.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Move the friends list to a subcollection under each user document.
Why it's wrong here
Subcollections are good for one-to-many, but for many-to-many relationships, a separate collection is often better.
- ✗
Migrate user profiles and friendships to Cloud SQL for relational capabilities.
Why it's wrong here
This is a complete database migration, not a schema redesign within Firestore.
- ✗
Limit the maximum size of the friends array to 1000 at the application level.
Why it's wrong here
This is an application constraint, not a scalable schema design.
- ✓
Create a new 'friendships' collection with documents containing user_id_1 and user_id_2 fields.
Why this is correct
A separate collection for relationships scales well and avoids large documents.
Clue confirmation
The clue word "best" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume subcollections (Option A) are the universal solution for nested data growth, but they fail to recognize that subcollections still tie writes to a parent document's write limit and do not solve the array-size problem; the correct approach is to normalize the relationship into a separate top-level collection.
Detailed technical explanation
How to think about this question
Firestore enforces a maximum document size of 1 MiB and a maximum write rate of 1 transaction per second per document. By storing friendships as individual documents in a 'friendships' collection, each write is a small, independent operation that can scale horizontally across many documents. A common pattern is to store two documents per friendship (one for each direction) or use a composite key like 'userA_userB' to enable efficient queries for a user's friends without scanning all documents.
KKey Concepts to Remember
- Read the scenario before looking for a memorised answer.
- Find the constraint that changes the correct option.
- Eliminate answers that are true in general but not in this case.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
An e-commerce site experiences heavy traffic on Black Friday and near-zero traffic during off-peak weeks. Rather than provisioning permanent large VMs, the team uses auto-scaling groups that add capacity automatically under load and reduce it overnight. Questions like this test whether you understand elasticity, availability zones, and cloud compute scaling patterns.
What to study next
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FAQ
Questions learners often ask
What does this PCDE question test?
Design and implement database schemas — This question tests Design and implement database schemas — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Create a new 'friendships' collection with documents containing user_id_1 and user_id_2 fields. — Option D is correct because it normalizes the friendship relationship into a separate 'friendships' collection, where each document represents a single bidirectional link between two users. This avoids unbounded document growth and write contention on user documents, as adding a friend only requires a small write to a new friendship document rather than updating a potentially large array. Firestore's 1 MiB document size limit and 1 write per second per document limit are no longer risk factors.
What should I do if I get this PCDE question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "best". Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jul 4, 2026
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