SQLAlchemy tips in Python web development

With the continuous development of Internet technology, Python web development online applications are gradually becoming more popular. SQLAlchemy is one of the most popular ORM (Object-Relational Mapping) frameworks in Python. The efficient and simple database operation method it provides not only realizes the seamless link between Python web development and database, but also is favored by many developers. This article will share some SQLAlchemy skills in Python web development to help developers complete project development more efficiently.

1. Use contextlib to ensure the closing of the session

ORM framework generally contains a session (Session) object. When a developer needs to interact with the database, a session object will be created and operated, but the session object must be closed after the operation is completed, otherwise some unexpected problems will occur.

In order to ensure the management of session objects, the contextlib context method can be used to temporarily modify system properties in Python. Using contextlib can ensure that the session is closed while avoiding overly cumbersome try/except processing. The code is as follows:

from contextlib import contextmanager
from sqlalchemy.orm import sessionmaker

engine = create_engine(DB_URL)
Session = sessionmaker(bind=engine)

@contextmanager
def session_scope():
    session = Session()
    try:
        yield session
        session.commit()
    except:
        session.rollback()
        raise
    finally:
        session.close()

The above code creates a session_scope context manager, and session_scope hosts the session object of SQLAlchemy. , which encapsulates database operations. In actual development, we can use the with syntax block to automatically manage the creation and destruction of sessions.

# 使用with自动管理session的创建与销毁
with session_scope() as session:
    # 实现对数据库的操作

2. Use Query Objects for efficient query

SQLAlchemy’s Query API provides a method to query the database programmatically. Query API has powerful filtering, grouping, sorting and other functions, which can help developers quickly and easily define query conditions.

Query Objects is the return value of Query and is a data type unique to SQLAlchemy. Query Objects is a chain structure, and query statements can be built by adding method chains. Query Objects can be used as a "query template" that can be reused in many different places, thus avoiding duplication of code for multiple similar queries.

# 查询books表中价格高于50的所有图书
books = session.query(Book).filter(Book.price > 50).all()

For complex queries, multiple methods can be used to build the query. The following example shows how to use the chain structure of Query Objects to implement grouping, sorting and limiting queries:

# 查询books表中，数量大于0，价格低于100元的所有书籍分页显示
books_query = session.query(Book).filter(Book.stock > 0).filter(Book.price < 100)
books_query = books_query.order_by(Book.price.desc())
books_query = books_query.limit(20).offset(0)
books = books_query.all()

Query Objects also supports many other query methods, such as count(), first(), etc., which can be used through Query View API documentation.

3. Use Laziness for performance optimization

When using Query Objects to query a large amount of data, sometimes it is necessary to use Laziness lazy loading to reduce the database load and improve query efficiency. Laziness refers to querying when the data is needed, rather than getting results immediately after each query.

The following is how to use Laziness:

# 查询users表中用户名以A开头的所有用户的ID
def get_user_ids(startswith):
    query = session.query(User.id).filter(User.username.startswith("A"))
    return query

The above code returns a Query Objects object, not a list.

Using this method avoids querying and storing the entire result set before retrieving all results. Break the query into small batches and load data only when needed. This can reduce the burden on the computer and improve the efficiency of Python web development when querying the database.

4. Use Hybrid Attributes to automatically count

In Python web development, processing queries on multiple tables at the same time usually requires writing many query statements to obtain data. Hybrid Attributes provide methods for handling multi-table relationships, with automatic counting and aggregation.

The following is a code example using Hybrid Attributes:

class User(Base):
    __tablename__ = 'users'
    id = Column(Integer(), primary_key=True)
    name = Column(String(255), nullable=False)
    messages = relationship('Message')

    @hybrid_property
    def num_messages(self):
        return len(self.messages)

    @num_messages.expression
    def num_messages(cls):
        return (
            select([func.count(Message.id)])
            .where(cls.id == Message.user_id)
            .label("num_messages")
        )

In this example, num_messages can automatically count how many messages a user has. The Query API can use this property as if it were a field in the database schema. And can be queried and filtered like other ordinary attributes. num_messages uses the characteristics of Hybrid Attributes and uses two definitions - one is a decorator defined by pure Python, and the other is a SQLalchemy expression definition.

In short, using SQLAlchemy for database operations in Python web development can help developers perform database operations more elegantly and improve development efficiency. This article introduces some SQLAlchemy skills in Python web development, hoping to help readers. For different situations, we should make full use of the rich API provided by SQLAlchemy as much as possible to avoid duplication of work and make Python web development more efficient.

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