Question

我有一个星型模式架构数据库，我想在SQLAlchemy中表示。现在我遇到了如何以最佳方式完成这项工作的问题。现在我有很多具有自定义连接条件的属性，因为数据存储在不同的表中。如果有可能重新使用不同事实表格的维度，那将是很好的但我还没弄清楚如何做得很好。

Answer 1

星型模式中的典型事实表包含对所有维度表的外键引用，因此通常不需要自定义连接条件 - 它们是从外键引用自动确定的。

例如，具有两个事实表的星型模式如下所示：

Base = declarative_meta()

class Store(Base):
    __tablename__ = 'store'

    id = Column('id', Integer, primary_key=True)
    name = Column('name', String(50), nullable=False)

class Product(Base):
    __tablename__ = 'product'

    id = Column('id', Integer, primary_key=True)
    name = Column('name', String(50), nullable=False)

class FactOne(Base):
    __tablename__ = 'sales_fact_one'

    store_id = Column('store_id', Integer, ForeignKey('store.id'), primary_key=True)
    product_id = Column('product_id', Integer, ForeignKey('product.id'), primary_key=True)
    units_sold = Column('units_sold', Integer, nullable=False)

    store = relation(Store)
    product = relation(Product)

class FactTwo(Base):
    __tablename__ = 'sales_fact_two'

    store_id = Column('store_id', Integer, ForeignKey('store.id'), primary_key=True)
    product_id = Column('product_id', Integer, ForeignKey('product.id'), primary_key=True)
    units_sold = Column('units_sold', Integer, nullable=False)

    store = relation(Store)
    product = relation(Product)

但是假设你想在任何情况下减少样板。我将在维度类的本地创建生成器，这些生成器在事实表上进行自我配置：

class Store(Base):
    __tablename__ = 'store'

    id = Column('id', Integer, primary_key=True)
    name = Column('name', String(50), nullable=False)

    @classmethod
    def add_dimension(cls, target):
        target.store_id = Column('store_id', Integer, ForeignKey('store.id'), primary_key=True)
        target.store = relation(cls)

在哪种情况下用法如下：

class FactOne(Base):
    ...

Store.add_dimension(FactOne)

但是，这有一个问题。假设您要添加的维度列是主键列，则映射器配置将失败，因为类需要在设置映射之前设置其主键。因此，假设我们使用声明式（您将在下面看到它有一个很好的效果），为了使这种方法起作用，我们必须使用instrument_declarative()函数而不是标准元类：

meta = MetaData()
registry = {}
def register_cls(*cls):
    for c in cls:
        instrument_declarative(c, registry, meta)

那么我们就会采取以下措施：

class Store(object):
    # ...

class FactOne(object):
    __tablename__ = 'sales_fact_one'

Store.add_dimension(FactOne)

register_cls(Store, FactOne)

如果您确实有合理的自定义连接条件，只要有一些模式来创建这些条件，您就可以使用add_dimension()生成：

class Store(object):
    ...

    @classmethod
    def add_dimension(cls, target):
        target.store_id = Column('store_id', Integer, ForeignKey('store.id'), primary_key=True)
        target.store = relation(cls, primaryjoin=target.store_id==cls.id)

但是如果你使用的是2.6，那么最后很酷的事情就是将add_dimension变成类装饰器。以下是清理所有内容的示例：

from sqlalchemy import *
from sqlalchemy.ext.declarative import instrument_declarative
from sqlalchemy.orm import *

class BaseMeta(type):
    classes = set()
    def __init__(cls, classname, bases, dict_):
        klass = type.__init__(cls, classname, bases, dict_)
        if 'metadata' not in dict_:
            BaseMeta.classes.add(cls)
        return klass

class Base(object):
    __metaclass__ = BaseMeta
    metadata = MetaData()
    def __init__(self, **kw):
        for k in kw:
            setattr(self, k, kw[k])

    @classmethod
    def configure(cls, *klasses):
        registry = {}
        for c in BaseMeta.classes:
            instrument_declarative(c, registry, cls.metadata)

class Store(Base):
    __tablename__ = 'store'

    id = Column('id', Integer, primary_key=True)
    name = Column('name', String(50), nullable=False)

    @classmethod
    def dimension(cls, target):
        target.store_id = Column('store_id', Integer, ForeignKey('store.id'), primary_key=True)
        target.store = relation(cls)
        return target

class Product(Base):
    __tablename__ = 'product'

    id = Column('id', Integer, primary_key=True)
    name = Column('name', String(50), nullable=False)

    @classmethod
    def dimension(cls, target):
        target.product_id = Column('product_id', Integer, ForeignKey('product.id'), primary_key=True)
        target.product = relation(cls)
        return target

@Store.dimension
@Product.dimension
class FactOne(Base):
    __tablename__ = 'sales_fact_one'

    units_sold = Column('units_sold', Integer, nullable=False)

@Store.dimension
@Product.dimension
class FactTwo(Base):
    __tablename__ = 'sales_fact_two'

    units_sold = Column('units_sold', Integer, nullable=False)

Base.configure()

if __name__ == '__main__':
    engine = create_engine('sqlite://', echo=True)
    Base.metadata.create_all(engine)

    sess = sessionmaker(engine)()

    sess.add(FactOne(store=Store(name='s1'), product=Product(name='p1'), units_sold=27))
    sess.commit()

SQLAlchemy中的星型模式

1 个答案: