# ! /usr/bin/env python
# -*- coding: utf-8 -*-
# login_frontend.py
""" Python 2.7.3
Cherrypy 3.2.2
PostgreSQL 9.1
psycopy2 2.4.5
SQLAlchemy 0.7.10
"""
I'm having a problem joining four tables in one Python/SQLAlchemy class. I'm trying this, so I can iterate the instance of this class, instead of the named tuple, which I get from joining tables with the ORM.
Why all of this? Because I already started that way and I came too far, to just leave it. Also, it has to be possible, so I want to know how it's done.
For this project (cherrypy web-frontend) I got an already completed module with the table classes. I moved it to the bottom of this post, because maybe it isn't even necessary for you.
The following is just one example of a joined multiple tables class attempt. I picked a simple case with more than only two tables and a junction table. Here I don't write into these joined tables, but it is necessary somewhere else. That's why classes would be a nice solution to this problem.
My attempt of a join class,
which is a combination of the given table classes module and the examples from these two websites:
-Mapping a Class against Multiple Tables
-SQLAlchemy: one classes – two tables
class JoinUserGroupPerson (Base):
persons = md.tables['persons']
users = md.tables['users']
user_groups = md.tables['user_groups']
groups = md.tables['groups']
user_group_person =(
join(persons, users, persons.c.id == users.c.id).
join(user_groups, users.c.id == user_groups.c.user_id).
join(groups, groups.c.id == user_groups.c.group_id))
__table__ = user_group_person
""" I expanded the redefinition of 'id' to three tables,
and removed this following one, since it made no difference:
users_id = column_property(users.c.id, user_groups.c.user_id)
"""
id = column_property(persons.c.id, users.c.id, user_groups.c.user_id)
groups_id = column_property(groups.c.id, user_groups.c.group_id)
groups_name = groups.c.name
def __init__(self, group_name, login, name, email=None, phone=None):
self.groups_name = group_name
self.login = login
self.name = name
self.email = email
self.phone = phone
def __repr__(self):
return(
"<JoinUserGroupPerson('%s', '%s', '%s', '%s', '%s')>" %(
self.groups_name, self.login, self.name, self.email, self.phone))
Different table accesses with this join class
This is how I tried to query this class in another module:
pg = sqlalchemy.create_engine(
'postgresql://{}:{}@{}:{}/{}'.
format(user, password, server, port, data))
Session = sessionmaker(bind=pg)
s1 = Session()
query = (s1.query(JoinUserGroupPerson).
filter(JoinUserGroupPerson.login==user).
order_by(JoinUserGroupPerson.id))
record = {}
for rowX in query:
for colX in rowX.__table__.columns:
record[column.name] = getattr(rowX,colX.name)
""" RESULT:
"""
Traceback (most recent call last):
File "/usr/local/lib/python2.7/dist-packages/cherrypy/_cprequest.py", line 656, in respond
response.body = self.handler()
File "/usr/local/lib/python2.7/dist-packages/cherrypy/lib/encoding.py", line 228, in __call__
ct.params['charset'] = self.find_acceptable_charset()
File "/usr/local/lib/python2.7/dist-packages/cherrypy/lib/encoding.py", line 134, in find_acceptable_charset
if encoder(encoding):
File "/usr/local/lib/python2.7/dist-packages/cherrypy/lib/encoding.py", line 86, in encode_string
for chunk in self.body:
File "XXX.py", line YYY, in ZZZ
record[colX.name] = getattr(rowX,colX.name)
AttributeError: 'JoinUserGroupPerson' object has no attribute 'user_id'
Then I checked the table attributes:
for rowX in query:
return (u'{}'.format(rowX.__table__.columns))
""" RESULT:
"""
['persons.id',
'persons.name',
'persons.email',
'persons.phone',
'users.id',
'users.login',
'user_groups.user_id',
'user_groups.group_id',
'groups.id',
'groups.name']
Then I checked, if the query or my class isn't working at all, by using a counter.
I got up to (count == 5), so the first two joined tables. But when I set the condition to (count == 6), I got the first error message again. AttributeError: 'JoinUserGroupPerson' object has no attribute 'user_id'.:
list = []
for rowX in query:
for count, colX in enumerate(rowX.__table__.columns):
list.append(getattr(rowX,colX.name))
if count == 5:
break
return (u'{}'.format(list))
""" RESULT:
"""
[4, u'user real name', None, None, 4, u'user']
""" which are these following six columns:
persons[id, name, email, phone], users[id, login]
"""
Then I checked each column:
list = []
for rowX in query:
for colX in rowX.__table__.columns:
list.append(colX)
return (u'{}'.format(list))
""" RESULT:
"""
[Column(u'id', INTEGER(), table=, primary_key=True, nullable=False, server_default=DefaultClause(, for_update=False)),
Column(u'name', VARCHAR(length=252), table=, nullable=False),
Column(u'email', VARCHAR(), table=),
Column(u'phone', VARCHAR(), table=),
Column(u'id', INTEGER(), ForeignKey(u'persons.id'), table=, primary_key=True, nullable=False),
Column(u'login', VARCHAR(length=60), table=, nullable=False),
Column(u'user_id', INTEGER(), ForeignKey(u'users.id'), table=, primary_key=True, nullable=False),
Column(u'group_id', INTEGER(), ForeignKey(u'groups.id'), table=, primary_key=True, nullable=False),
Column(u'id', INTEGER(), table=, primary_key=True, nullable=False),
Column(u'name', VARCHAR(length=60), table=, nullable=False)]
Then I tried another two direct accesses, which got me both KeyErrors for 'id' and 'persons.id':
for rowX in query:
return (u'{}'.format(rowX.__table__.columns['id'].name))
for rowX in query:
return (u'{}'.format(rowX.__table__.columns['persons.id'].name))
Conclusion
I tried a few other things, which were even more confusing. Since they didn't reveal any more information, I didn't add them. I don't see where my class is wrong.
I guess, somehow I must have set the class in a way, which would only correctly join the first two tables. But the join works at least partially, because when the 'user_groups' table was empty, I got an empty query as well.
Or maybe I did something wrong with the mapping of this 'user_groups' table. Since with the join some columns are double, they need an additional definition. And the 'user_id' is already part of the persons and users table, so I had to map it twice.
I even tried to remove the 'user_groups' table from the join, because it's in the relationships (with secondary). It got me a foreign key error message. But maybe I just did it wrong.
Admittedly, I even don't know why ...
rowX.__table__.columns # column names as table name suffix
... has different attribute names than ...
colX in rowX.__table__.columns # column names without table names
Extra Edits
Another thought! Would all of this be possible with inheritance? Each class has its own mapping, but then the user_groups class may be necessary. The joins had to be between the single classes instead. The init() and repr() still had to be redefined.
It probably has something to do with the 'user_groups' table, because I even couldn't join it with the 'groups' or 'users' table. And it always says, that the class object has no attribute 'user_id'. Maybe it's something about the many-to-many relationship.
Attachment
Here is the already given SQLAlchemy module, with header, without specific information about the database, and the classes of the joined tables:
#!/usr/bin/python
# vim: set fileencoding=utf-8 :
import sqlalchemy
from sqlalchemy import join
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import relationship, backref, column_property
pg = sqlalchemy.create_engine(
'postgresql://{}@{}:{}/{}'.format(user, host, port, data))
md = sqlalchemy.MetaData(pg, True)
Base = declarative_base()
""" ... following, three of the four joined tables.
UserGroups isn't necessary, so it wasn't part of the module.
And the other six classes shouldn't be important for this ...
"""
class Person(Base):
__table__ = md.tables['persons']
def __init__(self, name, email=None, phone=None):
self.name = name
self.email = email
self.phone = phone
def __repr__(self):
return(
"<Person(%s, '%s', '%s', '%s')>" %(
self.id, self.name, self.email, self.phone))
class Group(Base):
__table__ = md.tables['groups']
def __init__(self, name):
self.name = name
def __repr__(self):
return("<Group(%s, '%s')>" %(self.id, self.name))
class User(Base):
__table__ = md.tables['users']
person = relationship('Person')
groups = relationship(
'Group', secondary=md.tables['user_groups'], order_by='Group.id',
backref=backref('users', order_by='User.login'))
def __init__(self, person, login):
if isinstance(person, Person):
self.person = person
else:
self.id = person
self.login = login
def __repr__(self):
return("<User(%s, '%s')>" %(self.id, self.login))
Maybe the following script, which created the database, and also was already given, will prove useful here. As last part of it comes some test data - but between the columns are supposed to be tabs, no spaces. Because of that, this script also can be found as gist on github:
-- file create_str.sql
-- database creation script
-- central script for creating all database objects
-- set the database name
set strdbname logincore
c admin
BEGIN;
i str_roles.sql
COMMIT;
DROP DATABASE IF EXISTS :strdbname;
CREATE DATABASE :strdbname TEMPLATE template1 OWNER str_db_owner
ENCODING 'UTF8';
c :strdbname
SET ROLE str_db_owner;
BEGIN;
i str.sql
COMMIT;
RESET ROLE;
-- file str_roles.sql
-- create roles for the database
-- owner of the database objects
SELECT create_role('str_db_owner', 'NOINHERIT');
-- role for using
SELECT create_role('str_user');
-- make str_db_owner member in all relevant roles
GRANT str_user TO str_db_owner WITH ADMIN OPTION;
-- file str.sql
-- creation of database
-- prototypes
i str_prototypes.sql
-- domain for non empty text
CREATE DOMAIN ntext AS text CHECK (VALUE<>'');
-- domain for email addresses
CREATE DOMAIN email AS varchar(252) CHECK (is_email_address(VALUE));
-- domain for phone numbers
CREATE DOMAIN phone AS varchar(60) CHECK (is_phone_number(VALUE));
-- persons
CREATE TABLE persons (
id serial PRIMARY KEY
