我是数据库新手,我正在练习基本操作。我在我的机器上设置了mysql,python和sqlalchemy(巨大的胜利)。
我已经用Departments和Employees编写了这个基本模式(我知道,非常无聊)。我已经想出如何进行内连接(下面包含的代码,随意复制),但我似乎无法获得外连接。
from sqlalchemy import create_engine
from sqlalchemy import Column, Integer, String, Table, Text, Date
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import ForeignKey
from sqlalchemy.orm import relationship, backref, sessionmaker
from datetime import datetime
engine = create_engine('sqlite:///:memory:', echo=False)
Session = sessionmaker(bind=engine)
session = Session()
Base = declarative_base()
class Department(Base):
__tablename__ = 'department'
id = Column(Integer, primary_key=True)
name = Column(String(100))
def __init__(self, id, name):
self.id = id
self.name = name
class Emp(Base):
__tablename__ = 'employee'
id = Column(Integer, primary_key=True)
mgr_id = Column(Integer, ForeignKey('employee.id'))
dept_id = Column(Integer, ForeignKey('department.id'))
name = Column(String(100))
sal = Column(Integer)
doj = Column(Date)
def __init__(self,id,mgr_id,dept_id,name,sal,doj):
self.id = id
self.mgr_id = mgr_id
self.dept_id = dept_id
self.name = name
self.sal = sal
self.doj = doj
Base.metadata.create_all(engine)
session.add_all([Department(1,'HR'), \
Department(2,'Engineering'), \
Department(3,'Marketing'), \
Department(4,'Sales'), \
Department(5,'Logistics')])
session.add_all([Emp(1, None, 2,'Hash', 100, datetime(2012,01,01)), \
Emp(3, 2, 1, 'Privy', 50, datetime(2012,05,01)), \
Emp(4, 1, 1, 'Inno', 50, datetime(2012,05,01)), \
Emp(5, 2, 2, 'Anno', 80, datetime(2012,02,01)), \
Emp(6, 1, 2, 'Darl', 80, datetime(2012,02,11)), \
Emp(7, 1, 3, 'Pete', 70, datetime(2012,04,16)), \
Emp(8, 7, 3, 'Meme', 60, datetime(2012,07,26)), \
Emp(9, 2, 4, 'Tomiti', 70, datetime(2012,07,07)), \
Emp(10, 9, 4, 'Bhuti', 60, datetime(2012,8,24)), \
Emp(2, 1, 2, 'Robo', 100, datetime(2012,01,01))])
print '\nDepartment names'
for instance in session.query(Department).order_by(Department.id):
print instance.id,instance.name
print '\nEmployee names'
for instance in session.query(Emp).order_by(Emp.id):
print instance.id,instance.name,instance.doj
print '\nInner Join'
for d, e in session.query(Department, Emp).\
filter(Department.id==Emp.dept_id).\
all():
print d.name, e.name
print '\nOuter Join' # fails in next line
for d, e in session.query(Department, Emp).\
outerjoin(Department.id).\
all():
print d.name, e.name
这是打印输出错误:
nb-meagain:sqlalchemy me$ python joinsEtc.py
Department names
1 HR
2 Engineering
3 Marketing
4 Sales
5 Logistics
Employee names
1 Hash 2012-01-01
2 Robo 2012-01-01
3 Privy 2012-05-01
4 Inno 2012-05-01
5 Anno 2012-02-01
6 Darl 2012-02-11
7 Pete 2012-04-16
8 Meme 2012-07-26
9 Tomiti 2012-07-07
10 Bhuti 2012-08-24
Inner Join
Engineering Hash
Engineering Robo
HR Privy
HR Inno
Engineering Anno
Engineering Darl
Marketing Pete
Marketing Meme
Sales Tomiti
Sales Bhuti
Outer Join
Traceback (most recent call last):
File "joinsEtc.py", line 85, in <module>
outerjoin(Department.id).\
File "/Users/me/Library/Python/2.7/lib/python/site-packages/sqlalchemy/orm/query.py", line 1688, in outerjoin
from_joinpoint=from_joinpoint)
File "<string>", line 1, in <lambda>
File "/Users/me/Library/Python/2.7/lib/python/site-packages/sqlalchemy/orm/query.py", line 51, in generate
fn(self, *args[1:], **kw)
File "/Users/me/Library/Python/2.7/lib/python/site-packages/sqlalchemy/orm/query.py", line 1768, in _join
right_entity = onclause.property.mapper
AttributeError: 'ColumnProperty' object has no attribute 'mapper'
答案 0 :(得分:4)
首先,你的内连接很可能甚至不是INNER JOIN
,而是导致相同最终结果的WHERE
子句。您可以查看生成的SQL
以验证
现在,内部/外部联接看起来如下:
print '\nInner Join2'
for d, e in session.query(Department, Emp).join(Emp):
print d.name, e.name
print '\nOuter Join' # from Dep -> Emp
for d, e in session.query(Department, Emp).outerjoin(Emp):
print d.name, e and e.name # NOTE: e Might be None because of the OUTER JOIN
print '\nOuter Join2' # from Emp -> Dep
for e, d in session.query(Emp, Department).outerjoin(Department):
print e.name, d and d.name # NOTE: d Might be None because of the OUTER JOIN
可选强> 的 此外,为了获得完整的模型,您还应该让SA模型了解这种关系:
class Department(Base):
...
employees = relationship("Emp", backref="department")
...
class Emp(Base):
...
manager = relationship("Emp", backref="team", remote_side=[id])
...
在这种情况下,您可以执行以下操作:
the_boss = session.query(Emp).get(1)
print the_boss.name
print the_boss.team
assert the_boss.team[0].manager == the_boss
print the_boss.department