SQLAlchemy setup with FastAPI - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the SQLAlchemy base class.

FastAPI

from sqlalchemy.ext.declarative import [1]

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

Adeclarative_base

BSessionLocal

Ccreate_engine

DBase

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'Base' directly without importing declarative_base

Confusing create_engine with declarative_base

The declarative_base function creates a base class for model classes in SQLAlchemy.

2fill in blank

medium

Complete the code to create the SQLAlchemy engine with a SQLite database URL.

FastAPI

engine = create_engine('[1]', connect_args={"check_same_thread": False})

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

A"sqlite:///./test.db"

B"postgresql://user:pass@localhost/dbname"

C"mysql://user:pass@localhost/dbname"

D"oracle://user:pass@localhost/dbname"

Attempts:

3 left

💡 Hint

Common Mistakes

Using a PostgreSQL or MySQL URL instead of SQLite

Missing the triple slashes after 'sqlite:'

The SQLite database URL format is sqlite:///./filename.db. This example uses test.db in the current folder.

3fill in blank

hard

Fix the error in the session creation code by filling the blank.

FastAPI

SessionLocal = sessionmaker(autocommit=False, autoflush=False, bind=[1])

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

ABase

Bengine

CSessionLocal

Dcreate_engine

Attempts:

3 left

💡 Hint

Common Mistakes

Binding to Base instead of engine

Binding to the sessionmaker itself

The bind parameter must be the SQLAlchemy engine instance to connect sessions to the database.

4fill in blank

hard

Fill both blanks to define a FastAPI dependency that provides a database session and closes it after use.

FastAPI

def get_db():
    db = [1]()
    try:
        yield db
    finally:
        db.[2]()

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

ASessionLocal

Bengine

Cclose

Dcommit

Attempts:

3 left

💡 Hint

Common Mistakes

Using engine instead of SessionLocal to create session

Calling commit instead of close in finally block

The function creates a session from SessionLocal and closes it with close() after use.

5fill in blank

hard

Fill all three blanks to define a SQLAlchemy model class with a table name, id column, and a string column.

FastAPI

class User([1]):
    __tablename__ = '[2]'
    id = Column(Integer, primary_key=True, index=True)
    name = Column([3])

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

ABase

Busers

CString

DInteger

Attempts:

3 left

💡 Hint

Common Mistakes

Using Integer for the name column

Forgetting to inherit from Base

Using singular table name

The model class inherits from Base. The table name is users. The name column uses the String type.

Practice

1. What is the main purpose of SessionLocal in a FastAPI app using SQLAlchemy?

easy

A. To create a new database session for each request

B. To define the database schema

C. To connect directly to the database engine

D. To store user authentication data

Solution

1. Step 1: Understand the role of SessionLocal

   SessionLocal is a session factory that creates new database sessions for each request to ensure safe and isolated database operations.

2. Step 2: Differentiate from other components

   The database schema is defined by models, the engine connects to the database, and user data is unrelated to SessionLocal.

3. Final Answer:

   To create a new database session for each request -> Option A

4. Quick Check:

   SessionLocal creates new sessions per request [OK]

Hint: SessionLocal always means a new session per request [OK]

Common Mistakes:

• Confusing SessionLocal with engine
• Thinking SessionLocal defines schema
• Assuming SessionLocal stores user data

2. Which of the following is the correct way to create the SQLAlchemy engine in FastAPI?

easy

A. engine = create_engine('sqlite:///./test.db', connect_args={'check_same_thread': False})

B. engine = create_engine('sqlite:///:memory:')

C. engine = create_engine('postgresql://user:pass@localhost/db')

D. engine = create_engine('mysql://user@localhost/db')

Solution

1. Step 1: Identify the common FastAPI SQLite engine setup

   FastAPI tutorials often use SQLite with the URL 'sqlite:///./test.db' and the argument to allow multiple threads.

2. Step 2: Check options for correctness

   engine = create_engine('sqlite:///./test.db', connect_args={'check_same_thread': False}) matches the typical FastAPI SQLite setup with connect_args to avoid threading errors.

3. Final Answer:

   engine = create_engine('sqlite:///./test.db', connect_args={'check_same_thread': False}) -> Option A

4. Quick Check:

   SQLite engine with check_same_thread=False [OK]

Hint: SQLite needs check_same_thread=False in FastAPI [OK]

Common Mistakes:

• Omitting connect_args causing threading errors
• Using wrong database URL format
• Confusing in-memory with file-based SQLite

3. Given this FastAPI SQLAlchemy session usage, what will print(user.name) output?

from sqlalchemy.orm import Session

def get_user(db: Session, user_id: int):
    return db.query(User).filter(User.id == user_id).first()

user = get_user(db=session, user_id=1)
print(user.name)

medium

A. None

B. The name of the user with id 1

C. Raises AttributeError

D. Raises SQLAlchemyError

Solution

1. Step 1: Understand the query behavior

   The query filters User by id=1 and returns the first match or None if not found.

2. Step 2: Analyze the print statement

   If a user with id=1 exists, user.name prints the name; otherwise, user is None and accessing name would error.

3. Final Answer:

   The name of the user with id 1 -> Option B

4. Quick Check:

   Query returns user object [OK]

Hint: Query.first() returns object or None; here user exists [OK]

Common Mistakes:

• Assuming print outputs None without checking user
• Expecting an error without verifying user exists
• Confusing filter with filter_by syntax

4. Identify the error in this FastAPI SQLAlchemy session usage:

def create_user(db: Session, user: UserCreate):
    db_user = User(name=user.name, email=user.email)
    db.add(db_user)
    # Missing db.commit()
    return db_user

medium

A. User model is not imported

B. db.add() should be db.insert()

C. Missing call to db.commit() to save changes

D. Function should return None

Solution

1. Step 1: Check session usage for saving data

   Adding an object to the session requires calling db.commit() to persist changes to the database.

2. Step 2: Verify other parts of the code

   db.add() is correct, User model import is assumed, and returning the new user is expected.

3. Final Answer:

   Missing call to db.commit() to save changes -> Option C

4. Quick Check:

   db.commit() needed after db.add() [OK]

Hint: Always commit after adding to session [OK]

Common Mistakes:

• Forgetting db.commit() after db.add()
• Using db.insert() instead of db.add()
• Returning wrong type from function

5. You want to set up SQLAlchemy with FastAPI to support multiple database types (SQLite, PostgreSQL) using environment variables. Which approach correctly configures the engine and session?

hard

A. Use sessionmaker() without binding engine

B. Hardcode SQLite URL in create_engine and ignore env vars

C. Create engine without URL and pass URL to sessionmaker

D. Use DATABASE_URL env var, pass it to create_engine, then create SessionLocal with sessionmaker(bind=engine)

Solution

1. Step 1: Use environment variable for database URL

   Reading DATABASE_URL from environment allows flexible switching between databases.

2. Step 2: Create engine with the URL and bind sessionmaker

   Pass the URL to create_engine, then bind the engine to sessionmaker to create SessionLocal.

3. Final Answer:

   Use DATABASE_URL env var, pass it to create_engine, then create SessionLocal with sessionmaker(bind=engine) -> Option D

4. Quick Check:

   Env var URL + engine + sessionmaker(bind=engine) [OK]

Hint: Always bind engine to sessionmaker using env var URL [OK]

Common Mistakes:

• Hardcoding URLs reduces flexibility
• Not binding engine to sessionmaker causes errors
• Passing URL to sessionmaker instead of create_engine