MongoDB integration with Motor in FastAPI - Performance & Optimization

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Practice

1. What is the main benefit of using Motor with FastAPI for MongoDB operations?

easy

A. It automatically converts ObjectId to string.

B. It provides a graphical interface for MongoDB.

C. It allows asynchronous, non-blocking database calls.

D. It replaces the need for FastAPI routing.

Solution

1. Step 1: Understand Motor's role in FastAPI

   Motor is an async driver for MongoDB, enabling non-blocking calls in FastAPI apps.

2. Step 2: Identify the main benefit

   Non-blocking calls keep the app responsive and fast by not waiting for DB operations.

3. Final Answer:

   It allows asynchronous, non-blocking database calls. -> Option C

4. Quick Check:

   Motor = async, non-blocking calls [OK]

Hint: Motor means async MongoDB calls in FastAPI [OK]

Common Mistakes:

• Thinking Motor provides UI tools
• Assuming Motor auto-converts ObjectId
• Confusing Motor with FastAPI routing

2. Which of the following is the correct way to define a Motor client in FastAPI?

easy

A. client = motor.motor_asyncio.AsyncIOMotorClient('mongodb://localhost:27017')

B. client = MotorClient('mongodb://localhost:27017')

C. client = AsyncMongoClient('localhost:27017')

D. client = motor.MongoClient('mongodb://localhost:27017')

Solution

1. Step 1: Recall Motor client syntax

   The Motor async client is created using motor.motor_asyncio.AsyncIOMotorClient with the MongoDB URI.

2. Step 2: Check each option

   Only client = motor.motor_asyncio.AsyncIOMotorClient('mongodb://localhost:27017') uses the correct class and URI format for Motor.

3. Final Answer:

   client = motor.motor_asyncio.AsyncIOMotorClient('mongodb://localhost:27017') -> Option A

4. Quick Check:

   Motor client = AsyncIOMotorClient [OK]

Hint: Use AsyncIOMotorClient from motor.motor_asyncio [OK]

Common Mistakes:

• Using MotorClient instead of AsyncIOMotorClient
• Omitting 'motor.motor_asyncio' prefix
• Using synchronous MongoClient

3. Given this FastAPI async function using Motor, what will be the output?

async def get_user(db, user_id):
    user = await db.users.find_one({"_id": user_id})
    return str(user["_id"])

medium

A. Returns None if user_id is not found.

B. Returns the string representation of the user's ObjectId.

C. Raises a TypeError because ObjectId can't be converted to string.

D. Returns the user document as a dictionary.

Solution

1. Step 1: Analyze the async function

   The function awaits a find_one query by _id and then converts the _id field to string.

2. Step 2: Understand the output

   It returns the string form of the ObjectId, not the whole document or None.

3. Final Answer:

   Returns the string representation of the user's ObjectId. -> Option B

4. Quick Check:

   str(ObjectId) = string id [OK]

Hint: str() converts ObjectId to string safely [OK]

Common Mistakes:

• Expecting full user dict as output
• Thinking ObjectId can't be stringified
• Assuming None is returned if not found

4. Identify the error in this Motor usage inside FastAPI:

async def fetch_items(db):
    items = db.items.find()
    return await items.to_list(length=100)

medium

A. Missing await before db.items.find() call.

B. to_list() should not be awaited.

C. find() returns a cursor and must be awaited before to_list().

D. No error; code is correct.

Solution

1. Step 1: Understand Motor's find() behavior

   Motor's find() returns an async cursor immediately; no await needed here.

2. Step 2: Check to_list() usage

   to_list() is a coroutine and must be awaited to get the list of documents.

3. Final Answer:

   No error; code is correct. -> Option D

4. Quick Check:

   find() no await, to_list() await [OK]

Hint: Await to_list(), not find() [OK]

Common Mistakes:

• Awaiting find() call incorrectly
• Not awaiting to_list() causing runtime error
• Confusing synchronous and async cursor methods

5. You want to return a list of users from MongoDB in FastAPI, but each user's _id must be a string, not ObjectId. Which code snippet correctly does this?

hard

A. users = await db.users.find().to_list(100)
return [{**u, '_id': str(u['_id'])} for u in users]

B. users = await db.users.find().to_list(100)
return [u for u in users if str(u['_id'])]

C. users = await db.users.find().to_list(100)
return [str(u) for u in users]

D. users = await db.users.find().to_list(100)
return [u['_id'] for u in users]

Solution

1. Step 1: Fetch users as list of dicts

   await db.users.find().to_list(100) returns list of user dicts with ObjectId _id fields.

2. Step 2: Convert each user's _id to string

   Use list comprehension to copy each dict and replace _id with str(u['_id']).

3. Final Answer:

   users = await db.users.find().to_list(100) return [{**u, '_id': str(u['_id'])} for u in users] -> Option A

4. Quick Check:

   Convert ObjectId to string per user [OK]

Hint: Use dict unpacking and str() on _id in list comprehension [OK]

Common Mistakes:

• Returning ObjectId directly without conversion
• Trying to convert whole user dict to string
• Filtering users instead of converting _id