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of 36What is SQLAlchemy and what are its main components?
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SQLAlchemy is a Python SQL toolkit and Object-Relational Mapping (ORM) library that provides a full suite of well-known enterprise-level persistence patterns. It has two main components:
- SQLAlchemy Core: A schema-centric model that provides a Pythonic way of working with databases using SQL expressions
- SQLAlchemy ORM: An object-relational mapper built on top of Core that allows you to work with database records as Python objects
The main benefits include database abstraction, connection pooling, transaction management, and a powerful query API that works across different database engines.
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What's the difference between SQLAlchemy Core and ORM?
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SQLAlchemy Core is a lower-level, schema-centric approach that works directly with tables, columns, and SQL expressions. It's closer to raw SQL and offers more control.
SQLAlchemy ORM is a higher-level, object-centric approach that maps database tables to Python classes and rows to object instances.
# Core approach
from sqlalchemy import text
result = connection.execute(text("SELECT * FROM users WHERE id = :user_id"), {"user_id": 1})
# ORM approach
user = session.query(User).filter(User.id == 1).first()
Core is typically faster and more explicit, while ORM provides more abstraction and is easier for complex object relationships.
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Explain the concept of a SQLAlchemy Session.
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A Session in SQLAlchemy ORM is the primary interface for persistence operations. It represents a "workspace" for your objects and acts as a holding zone for all objects you've loaded or created until you commit the changes to the database.
Key characteristics:
- Identity Map: Ensures one object instance per database row per session
- Unit of Work: Tracks changes and flushes them to database in transactions
- Transaction Management: Handles database transactions automatically
from sqlalchemy.orm import sessionmaker
Session = sessionmaker(bind=engine)
session = Session()
# Add new object
user = User(name="John")
session.add(user)
session.commit() # Persists to database
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What is the difference between `add()`, `merge()`, and `add_all()` in SQLAlchemy?
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add(): Adds a single new object to the session. If object already exists, raises an error.merge(): Merges the state of an object into the session. If object exists, updates it; if not, creates new one.add_all(): Adds multiple objects to the session at once.
# add() - for new objects
session.add(User(name="Alice"))
# merge() - for existing or uncertain objects
user = session.merge(User(id=1, name="Updated Alice"))
# add_all() - for multiple objects
session.add_all([User(name="Bob"), User(name="Charlie")])
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How do you define a basic SQLAlchemy model?
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A SQLAlchemy model is defined by creating a class that inherits from a declarative base and includes table metadata:
from sqlalchemy import Column, Integer, String, create_engine
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import sessionmaker
Base = declarative_base()
class User(Base):
__tablename__ = 'users'
id = Column(Integer, primary_key=True)
name = Column(String(50), nullable=False)
email = Column(String(100), unique=True)
def __repr__(self):
return f"<User(name='{self.name}', email='{self.email}')>"
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Explain the difference between `get()` and `query().filter().first()`.
Explain different types of relationships in SQLAlchemy.
What is lazy loading and what are the different loading strategies?
How do you handle database migrations in SQLAlchemy?
What is the N+1 query problem and how do you solve it?
Explain the difference between `flush()` and `commit()`.
What are SQLAlchemy events and how do you use them?
How do you implement database connection pooling?
What is the Unit of Work pattern in SQLAlchemy?
What are SQLAlchemy Core expressions and how do they differ from ORM queries?
How do you implement database transactions with rollback handling?
What is the difference between `declarative_base()` and the newer declarative mapping styles?
How do you implement soft deletes in SQLAlchemy?
How do you implement database-level constraints in SQLAlchemy?
What is the purpose of `cascade` options in relationships?
How do you implement database sharding with SQLAlchemy?
How do you implement multi-database support in SQLAlchemy?
What are some common SQLAlchemy anti-patterns and how do you avoid them?
How do you implement optimistic locking in SQLAlchemy?
How do you handle bulk operations efficiently in SQLAlchemy?
What is the purpose of `scoped_session` and when should you use it?
How do you implement custom column types in SQLAlchemy?
Explain the concept of SQLAlchemy mixins and their use cases.
What are hybrid properties and computed columns?
What are association objects and when should you use them?
How do you implement connection retry logic and error handling?
How do you optimize SQLAlchemy queries for performance?
Explain the concept of SQLAlchemy plugins and extensions.
How do you implement database connection retry and failover strategies?
What are SQLAlchemy performance best practices for large-scale applications?
How do you implement database-agnostic applications with SQLAlchemy?
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SQLAlchemy cheatsheet
SQLAlchemy Interview Cheat Sheet
- Summary01
- 1. Installation & Basic Setup02
- 2. Core Components03
- 3. Declarative Mapping04
- 4. CRUD Operations05
- 5. Relationships06
- 6. Query Operations07
- 7. Joins08
- 8. Transactions09
- 9. Advanced Features10
- 10. Performance Optimization11
- 11. Common Interview Patterns12
- + 3 more inside
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