使用混合¶

将相对简单的SQL表达式链接到类的最简单和最灵活的方法是使用所谓的“混合属性”，如Hybrid Attributes部分中所述。该混合提供了一种既适用于Python级别又适用于SQL表达级别的表达式。例如，下面我们映射一个类User，它包含属性firstname和lastname，并且包含一个混合，它将为我们提供fullname，这是两个字符串的连接：

from sqlalchemy.ext.hybrid import hybrid_property

class User(Base):
    __tablename__ = 'user'
    id = Column(Integer, primary_key=True)
    firstname = Column(String(50))
    lastname = Column(String(50))

    @hybrid_property
    def fullname(self):
        return self.firstname + " " + self.lastname

上面，fullname属性在实例和类级别都被解释，以便它可以从实例中获得：

some_user = session.query(User).first()
print(some_user.fullname)

以及在查询中可用：

some_user = session.query(User).filter(User.fullname == "John Smith").first()

字符串连接的例子很简单，Python表达式可以在实例和类级别双重使用。通常，必须将SQL表达式与Python表达式区分开来，这可以使用hybrid_property.expression()来实现。下面我们通过Python中的if语句和SQL表达式的sql.expression.case()构造来说明条件需要存在于混合内部的情况：

from sqlalchemy.ext.hybrid import hybrid_property
from sqlalchemy.sql import case

class User(Base):
    __tablename__ = 'user'
    id = Column(Integer, primary_key=True)
    firstname = Column(String(50))
    lastname = Column(String(50))

    @hybrid_property
    def fullname(self):
        if self.firstname is not None:
            return self.firstname + " " + self.lastname
        else:
            return self.lastname

    @fullname.expression
    def fullname(cls):
        return case([
            (cls.firstname != None, cls.firstname + " " + cls.lastname),
        ], else_ = cls.lastname)

使用column_property ¶

orm.column_property()函数可用于以类似于定期映射的Column的方式映射SQL表达式。使用这种技术，该属性在加载时与所有其他列映射属性一起加载。这在某些情况下比混合使用更有优势，因为该值可以与对象的父行同时加载，特别是当表达式链接到其他表时（通常作为相关子查询）来访问通常不会在已经加载的对象上可用的数据。

Disadvantages to using orm.column_property() for SQL expressions include that the expression must be compatible with the SELECT statement emitted for the class as a whole, and there are also some configurational quirks which can occur when using orm.column_property() from declarative mixins.

我们的“全名”示例可以使用orm.column_property()表示如下：

from sqlalchemy.orm import column_property

class User(Base):
    __tablename__ = 'user'
    id = Column(Integer, primary_key=True)
    firstname = Column(String(50))
    lastname = Column(String(50))
    fullname = column_property(firstname + " " + lastname)

相关的子查询也可以使用。下面我们使用select()构造创建一个SELECT，它将特定User可用的Address对象的计数链接在一起：

from sqlalchemy.orm import column_property
from sqlalchemy import select, func
from sqlalchemy import Column, Integer, String, ForeignKey

from sqlalchemy.ext.declarative import declarative_base

Base = declarative_base()

class Address(Base):
    __tablename__ = 'address'
    id = Column(Integer, primary_key=True)
    user_id = Column(Integer, ForeignKey('user.id'))

class User(Base):
    __tablename__ = 'user'
    id = Column(Integer, primary_key=True)
    address_count = column_property(
        select([func.count(Address.id)]).\
            where(Address.user_id==id).\
            correlate_except(Address)
    )

在上面的例子中，我们定义了如下所示的select()结构：

select([func.count(Address.id)]).\
    where(Address.user_id==id).\
    correlate_except(Address)

The meaning of the above statement is, select the count of Address.id rows where the Address.user_id column is equated to id, which in the context of the User class is the Column named id (note that id is also the name of a Python built in function, which is not what we want to use here - if we were outside of the User class definition, we’d use User.id).

select.correlate_except()指示表明可以从FROM列表中省略此select()的FROM子句中的每个元素（即，与封闭的SELECT针对User的声明）除了与Address对应的声明之外。This isn’t strictly necessary, but prevents Address from being inadvertently omitted from the FROM list in the case of a long string of joins between User and Address tables where SELECT statements against Address are nested.

如果导入问题阻止column_property()与该类内联定义，则可以在两者都配置后将其分配给类。在Declarative中，这具有调用Mapper.add_property()在事实之后添加其他属性的效果：

User.address_count = column_property(
        select([func.count(Address.id)]).\
            where(Address.user_id==User.id)
    )

对于多对多关系，使用and_()将关联表的字段连接到关系中的两个表，这里用经典映射来说明：

from sqlalchemy import and_

mapper(Author, authors, properties={
    'book_count': column_property(
                        select([func.count(books.c.id)],
                            and_(
                                book_authors.c.author_id==authors.c.id,
                                book_authors.c.book_id==books.c.id
                            )))
    })

使用普通描述符¶

如果SQL查询比orm.column_property()或hybrid_property可以提供的更精细，则可以使用作为属性访问的常规Python函数，假设该表达式只需要在已经加载的实例上可用。该函数使用Python自己的@property修饰器进行修饰，以将其标记为只读属性。Within the function, object_session() is used to locate the Session corresponding to the current object, which is then used to emit a query:

from sqlalchemy.orm import object_session
from sqlalchemy import select, func

class User(Base):
    __tablename__ = 'user'
    id = Column(Integer, primary_key=True)
    firstname = Column(String(50))
    lastname = Column(String(50))

    @property
    def address_count(self):
        return object_session(self).\
            scalar(
                select([func.count(Address.id)]).\
                    where(Address.user_id==self.id)
            )

普通描述符方法作为最后的手段是有用的，但在通常情况下，混合性和列属性方法的性能较差，因为它需要在每次访问时发出SQL查询。

SQL表达式作为映射的属性¶

使用混合¶

使用column_property ¶

使用普通描述符¶