How do you model many-to-many relationships in MongoDB vs SQL?

Question

"Users can bookmark questions. How do you model this in both databases?"

Interview OS Community · Accepted Answer

## SQL: Junction table
```sql
CREATE TABLE bookmarks (
  user_id TEXT REFERENCES users(id),
  question_id TEXT REFERENCES questions(id),
  created_at TIMESTAMP DEFAULT NOW(),
  PRIMARY KEY (user_id, question_id)
);
```

## MongoDB: Separate collection (recommended)
```javascript
{ userId: "u1", questionId: "q1", createdAt: ISODate() }
db.bookmarks.createIndex({ userId: 1, createdAt: -1 });
db.bookmarks.createIndex({ questionId: 1 });
```

Embedding arrays (`user.bookmarks: ["q1", "q2"]`) works for small lists but hits 16MB doc limit.

| Approach | MongoDB | SQL |
|----------|---------|-----|
| Few per doc | Embed array | Junction table |
| Many/unbounded | Separate collection | Junction table |
| Need metadata | Separate collection | Junction table |

Interview OS

How do you model many-to-many relationships in MongoDB vs SQL?

Question Details

Suggested Solution

SQL: Junction table

MongoDB: Separate collection (recommended)

Discussion (0)

Approach	MongoDB	SQL
Few per doc	Embed array	Junction table
Many/unbounded	Separate collection	Junction table
Need metadata	Separate collection	Junction table