[sqlalchemy] declarative polymorphic inheritance abstraction problem

[Chris Withers]

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

--
You received this message because you are subscribed to the Google Groups 
sqlalchemy group.
To unsubscribe from this group and stop receiving emails from it, send an email 
to sqlalchemy+unsubscr...@googlegroups.com.
To post to this group, send email to sqlalchemy@googlegroups.com.
Visit this group at http://groups.google.com/group/sqlalchemy.
For more options, visit https://groups.google.com/groups/opt_out.

Re: [sqlalchemy] declarative polymorphic inheritance abstraction problem

[Michael Bayer]

On Feb 10, 2014, at 2:36 PM, Chris Withers ch...@simplistix.co.uk wrote:

 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:

if you poke around with print statements or pdb, you can see that the “id” 
declared_attr is run only once, for Content.   this is because Content is 
mapped and your “id” declared_attr function is replaced with the mapped 
attribute “id”.

If you make a mixin like HasId and put it there, it still doesn’t work.   
Declarative finds the HasId mixin and the “id” function, but then when it 
resolves what “Project.id” really is, it again just calls getattr() and doesn’t 
call your function.   

Why’s it like that?  I don’t know, but I’m 99% sure at least three tests will 
fail if I change it, lets try:

Code like this:

class HasId(object):
@declared_attr
def id(cls):
if cls.__name__=='Content':
return Column('id', Integer, primary_key=True)
else:
return Column('id', Integer, ForeignKey('content.id'), 
primary_key=True)

class Content(HasId, Base):

content_type = Column(String(20))

@declared_attr
def __tablename__(cls):
return cls.__name__.lower()

class Project(Content):
currency  = Column(String(3))
target = Column(Integer)
current = Column(Integer)

so lets tell declarative to pull off of HasId.id when it scans “Project”, 
instead of getattr(Project, “id”):

--- a/lib/sqlalchemy/ext/declarative/base.py
+++ b/lib/sqlalchemy/ext/declarative/base.py
@@ -126,7 +126,7 @@ def _as_declarative(cls, classname, dict_):
 on declarative mixin classes.)
 elif isinstance(obj, declarative_props):
 dict_[name] = ret = \
-column_copies[obj] = getattr(cls, name)
+column_copies[obj] = getattr(base, name)
 if isinstance(ret, (Column, MapperProperty)) and \
 ret.doc is None:
 ret.doc = obj.__doc__

your test case works, but lets see the tests - six errors and four failures: 
https://gist.github.com/zzzeek/8925277

So we can’t do that.You know I love the mixins/declared_attr thing, but 
it’s hard.  There’s craploads of tests for it and the amount of decisions we’ve 
packed in there is already pretty intricate.

There’s some other variants that reduce the number of test failures, like this:

diff --git a/lib/sqlalchemy/ext/declarative/base.py 
b/lib/sqlalchemy/ext/declarative/base.py
index 4fda9c7..b74809e 100644
--- a/lib/sqlalchemy/ext/declarative/base.py
+++ b/lib/sqlalchemy/ext/declarative/base.py
@@ -126,7 +126,7 @@ def _as_declarative(cls, classname, dict_):
 on declarative mixin classes.)
 elif isinstance(obj, declarative_props):
 dict_[name] = ret = \
-column_copies[obj] = getattr(cls, name)
+column_copies[obj] = obj.__get__(obj, cls)
 if isinstance(ret, (Column, MapperProperty)) and \
 ret.doc is None:
 ret.doc = obj.__doc__


but still lots of failures; there’s a lot of use cases that just expect 
@declared_attr to hit once, then replace itself on the mapped class.

So the next idea is to just make a whole different command to do this, I 
sketched that up here:  www.sqlalchemy.org/trac/ticket/2952  

but I’m not really sure what I want to do with that.   mixins work great for 
single classes but within a hierarchy not so much, and adding more options 
makes me worried that we’re just adding flags rather than fixing the central 
idea somehow.  




signature.asc
Description: Message signed with OpenPGP using GPGMail