On 11/07/2016 05:43 PM, de...@devinfee.com wrote:
When using concrete table inheritance in the complete example below:
1. why must I return an empty `Column(Integer)` instead of `None`, or
receive the following error: /"ArgumentError: Mapper
AbstractConcreteBase works in an intricate way in order to get this very
unique effect of a class that is mapped, then has it's mapping changed
to be something totally different once all the descendant classes have
been produced. For this reason, it is more brittle than an ordinary
inheriting base class.
In this case, there are four things being done that are not expected by
declarative.
The first is that RefClass.__tablename__ returns a non-None value for
the RefClass class itself. This has the effect of a "refclass" table
being added to the Base.metadata which is not how AbstractConcreteBase
works; the "table" to be mapped will ultimately be a big SELECT
statement that is a UNION of all the descendant tables. So you don't
want a "refclass" table if you're using AbstractConcreteBase.
The second is that the @declared_attr.cascading feature, which is fairly
recent, expects that the attribute is being used to map columns in all
cases, and it also expects that it is used on a mixin class only. So in
this case, it's called only once, it returns None which immediately
replaces RefClass.id, and it's gone. "cascading" here should either
work on non-mixins or more likely for now report that it's being
mis-used if on a non-mixin,
https://bitbucket.org/zzzeek/sqlalchemy/issues/3847/declared_attrcascading-does-not-warn-error
is added. The reason you are forced to refer to RefClass by name is
because the id() def is called before RefClass exists as a symbol in
your program.
These two behaviors work together in that one you have __tablename__
return None for RefClass, you no longer need to play those same games
with the id() attribute, and the whole thing works.
The third is that @declared_attr overall works completely differently
when it returns a non-Column/ mapped value like "None" vs. a
column/mapper attribute. The value None can't be used in a mapping so
it's assumed that you want to set up RefClass.id = None, that blows away
any chance of the attribute being called for ARefClass. The reason
this is usually never a problem is because the parent class is either a
mixin/__abstract__, in which case it's not called, or a mapped class, in
which case it's called and it would return a Column. But here, we're
using the the very special / edge-case / brittle AbstractConcreteBase
class, which sort of acts like both, and returning None here totally
throws it off. So you need to return a column like it has in the
example at
http://docs.sqlalchemy.org/en/latest/orm/extensions/declarative/api.html?highlight=abstractconcretebase#sqlalchemy.ext.declarative.AbstractConcreteBase.
The Column is thrown away for RefClass because RefClass gets mapped to
a non-Table object later on. I'm not sure how to make this more
intuitive at the moment but I added
https://bitbucket.org/zzzeek/sqlalchemy/issues/3848/returning-none-or-anything-from.
The fourth issue is that AbstractConcreteBase is only useful for
polymorphic loading, and for it to be worth the effort here you need
concrete=True and polymorphic discriminators on your classes. If you
aren't going for polymorphic loading using a giant UNION statement (see
example below), don't use AbstractConcreteBase, just use a normal base
with __abstract__ = True.
class RefClass(AbstractConcreteBase, Base):
@declared_attr
def __tablename__(cls):
if cls.__name__ == 'RefClass':
return None
return cls.__name__.lower()
@declared_attr
def id(cls):
if cls.__name__ == 'RefClass':
return Column(Integer)
column_name = '{}.id'.format(cls.ref.__tablename__)
return Column(ForeignKey(column_name), primary_key=True)
class ARefClass(RefClass):
ref = AClass
__mapper_args__ = {"concrete": True, "polymorphic_identity": 'a'}
class BRefClass(RefClass):
ref = BClass
__mapper_args__ = {"concrete": True, "polymorphic_identity": 'b'}
output of db.query(RefClass).all():
SELECT pjoin.id AS pjoin_id, pjoin.type AS pjoin_type
FROM (SELECT brefclass.id AS id, 'b' AS type
FROM brefclass UNION ALL SELECT arefclass.id AS id, 'a' AS type
FROM arefclass) AS pjoin
Mapper|ARefClass|arefclass could not assemble any primary key
columns for mapped table 'arefclass'"/
2. why must I reference the class by name and not type in
`@declarad_attr`, unlike in a `classmethod`?
3. how does the `__tablename__` `declaredattr` actually behave any
differently than the `id` `declaradattr`?
from sqlalchemy import (
Column,
ForeignKey,
Integer,
create_engine,
)
from sqlalchemy.ext.declarative import (
AbstractConcreteBase,
declared_attr,
declarative_base,
has_inherited_table,
)
from sqlalchemy.orm import Session
Base = declarative_base()
class AClass(Base):
__tablename__ = 'aclass'
id = Column(Integer, primary_key=True)
class BClass(Base):
__tablename__ = 'bclass'
id = Column(Integer, primary_key=True)
class RefClass(AbstractConcreteBase, Base):
@declared_attr
def __tablename__(cls):
return cls.__name__.lower()
@declared_attr.cascading
def id(cls):
if cls.__name__ != 'RefClass':
column_name = '{}.id'.format(cls.ref.__tablename__)
return Column(ForeignKey(column_name), primary_key=True)
else:
# return # Fails as described in Q1
return Column(Integer)
class ARefClass(RefClass):
ref = AClass
class BRefClass(RefClass):
ref = BClass
engine = create_engine('sqlite://', echo=True)
Base.metadata.bind = engine
Base.metadata.create_all()
db = Session(engine)
Thanks!
Devin
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http://www.sqlalchemy.org/
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