Hi All,
I'm trying to be efficient with my code, but it seems to tripping
SQLAlchemy up.
So, basically I want to have a schema with two tables, content and
project, to represent two three types of object:
- article has a set of fields as found in the 'content' table
- project has fields that are the set union of the 'content' and
'project' tables' columns
- content is a plain content type, like article, and will likely never
be instantiated, but just in case...
- in future, I may want to add more tables/content types where the
fields are like project; a set union of the 'content' and 'project'
tables' columns
So, I tried to be clever and start off with the following content
class/table:
class Content(Base):
content_type = Column(String(20))
@declared_attr
def id(cls):
if cls.__name__=='Content':
return Column(Integer, primary_key=True)
else:
return Column(Integer, ForeignKey('content.id'),
primary_key=True)
@declared_attr
def __tablename__(cls):
return cls.__name__.lower()
@declared_attr
def __mapper_args__(cls):
args = dict(polymorphic_identity = cls.__name__.lower())
if cls.__name__=='Content':
args['polymorphic_on'] = cls.content_type
return args
...with the following two classes associated with it:
class Article(Content):
@declared_attr
def __tablename__(cls):
return None
class Project(Content):
currency = Column(String(3))
target = Column(Integer)
current = Column(Integer)
...but I get the following on import:
project.py", line 5, in <module>
class Project(Content):
File
"/Users/chris/buildout-eggs/SQLAlchemy-0.9.1-py2.7-macosx-10.5-x86_64.egg/sqlalchemy/ext/declarative/api.py",
line 53, in __init__
_as_declarative(cls, classname, cls.__dict__)
File
"/Users/chris/buildout-eggs/SQLAlchemy-0.9.1-py2.7-macosx-10.5-x86_64.egg/sqlalchemy/ext/declarative/base.py",
line 322, in _as_declarative
mt.map()
File
"/Users/chris/buildout-eggs/SQLAlchemy-0.9.1-py2.7-macosx-10.5-x86_64.egg/sqlalchemy/ext/declarative/base.py",
line 405, in map
**mapper_args
File "<string>", line 2, in mapper
File
"/Users/chris/buildout-eggs/SQLAlchemy-0.9.1-py2.7-macosx-10.5-x86_64.egg/sqlalchemy/orm/mapper.py",
line 593, in __init__
self._configure_inheritance()
File
"/Users/chris/buildout-eggs/SQLAlchemy-0.9.1-py2.7-macosx-10.5-x86_64.egg/sqlalchemy/orm/mapper.py",
line 900, in _configure_inheritance
self.local_table)
File "<string>", line 2, in join_condition
File
"/Users/chris/buildout-eggs/SQLAlchemy-0.9.1-py2.7-macosx-10.5-x86_64.egg/sqlalchemy/sql/selectable.py",
line 651, in _join_condition
"between '%s' and '%s'.%s" % (a.description, b.description, hint))
sqlalchemy.exc.NoForeignKeysError: Can't find any foreign key
relationships between 'content' and 'project'.
If I change the id method to be decorated by classproperty rather than
declared_attr, the error changes to:
article.py", line 3, in <module>
from .content import Content
content.py", line 7, in <module>
class Content(Base):
File
"/Users/chris/buildout-eggs/SQLAlchemy-0.9.1-py2.7-macosx-10.5-x86_64.egg/sqlalchemy/ext/declarative/api.py",
line 53, in __init__
_as_declarative(cls, classname, cls.__dict__)
File
"/Users/chris/buildout-eggs/SQLAlchemy-0.9.1-py2.7-macosx-10.5-x86_64.egg/sqlalchemy/ext/declarative/base.py",
line 322, in _as_declarative
mt.map()
File
"/Users/chris/buildout-eggs/SQLAlchemy-0.9.1-py2.7-macosx-10.5-x86_64.egg/sqlalchemy/ext/declarative/base.py",
line 405, in map
**mapper_args
File "<string>", line 2, in mapper
File
"/Users/chris/buildout-eggs/SQLAlchemy-0.9.1-py2.7-macosx-10.5-x86_64.egg/sqlalchemy/orm/mapper.py",
line 599, in __init__
self._configure_pks()
File
"/Users/chris/buildout-eggs/SQLAlchemy-0.9.1-py2.7-macosx-10.5-x86_64.egg/sqlalchemy/orm/mapper.py",
line 1189, in _configure_pks
(self, self.mapped_table.description))
sqlalchemy.exc.ArgumentError: Mapper Mapper|Content|content could not
assemble any primary key columns for mapped table 'content'
What's going wrong here?
cheers,
Chris
--
Simplistix - Content Management, Batch Processing & Python Consulting
- http://www.simplistix.co.uk
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