Re: [sqlalchemy] nested inheritance / polymorphic relationships
On Jun 4, 2013, at 1:55 AM, Amir Elaguizy aelag...@gmail.com wrote:
Hi there,
I have a tree that looks like this, reflected via polymorphic inheritance:
what do we mean reflected here, are you reflecting tables from the database,
that is,
http://docs.sqlalchemy.org/en/rel_0_8/core/schema.html#metadata-reflection ?
That works great, like:
class BaseModel(db.Model): # Table A in diagram
__tablename__ = entities
id = db.Column(db.BigInteger, primary_key=True, nullable=False,
server_default=func.nextval('guid_seq'))
type_id = db.Column(db.SmallInteger, db.ForeignKey(EntityTypesModel.id))
__mapper_args__ = {
'polymorphic_identity':'entity',
'polymorphic_on':type_id,
'with_polymorphic':'*'
}
this is why I question the word reflected because I don't see you using
reflection there.
class BrandModel(BaseModel): # Table B, C, D in diagram
__tablename__ = 'brands'
id = db.Column(db.BigInteger, db.ForeignKey(StufffModel.id),
primary_key=True, nullable=False)
name = db.Column(db.String, nullable=False)
__mapper_args__ = {
'polymorphic_identity':ET_BRAND,
}
Im confused by this as well - are you saying that you map the same class to B,
C, and D rows? That would be unusual. It wouldn't work at all on the
persistence side as SQLAlchemy could not know which of B, C, or D you wish for
a particular BrandModel to be persisted towards.
The problem is I need to reflect something more like this:
A
/ | \
B C D
/ \
EF
Where D is not only a polymorphic child of A but also the polymorphic parents
of E F.
It seems like I have to choose, D can either be a polymorphic child or it can
be a parent - it can't be both.
Do I have any options here?
SQLAlchemy can represent inheritance hierarchies of any depth.However,
because you are assigning a single subclass to all of B, C, and D that might be
why there's an issue here, you'd need to assign a distinct subclass of
BaseModel to at least D, and then another subclass of D_Model to handle E and
F.
Preferably, you'd produce distinct classes for all six tables.
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Re: [sqlalchemy] nested inheritance / polymorphic relationships
I didn't intend to use the word reflected in the Programming sense, I meant
in the traditional sense: is represented by.
That sentence was confusing, sorry!
I was saying class B, C, and D are all defined using that same pattern.
They each have their own class.
Amir
On Tuesday, June 4, 2013, Michael Bayer wrote:
On Jun 4, 2013, at 1:55 AM, Amir Elaguizy aelag...@gmail.comjavascript:;
wrote:
Hi there,
I have a tree that looks like this, reflected via polymorphic
inheritance:
what do we mean reflected here, are you reflecting tables from the
database, that is,
http://docs.sqlalchemy.org/en/rel_0_8/core/schema.html#metadata-reflection?
That works great, like:
class BaseModel(db.Model): # Table A in diagram
__tablename__ = entities
id = db.Column(db.BigInteger, primary_key=True, nullable=False,
server_default=func.nextval('guid_seq'))
type_id = db.Column(db.SmallInteger,
db.ForeignKey(EntityTypesModel.id))
__mapper_args__ = {
'polymorphic_identity':'entity',
'polymorphic_on':type_id,
'with_polymorphic':'*'
}
this is why I question the word reflected because I don't see you using
reflection there.
class BrandModel(BaseModel): # Table B, C, D in diagram
__tablename__ = 'brands'
id = db.Column(db.BigInteger, db.ForeignKey(StufffModel.id),
primary_key=True, nullable=False)
name = db.Column(db.String, nullable=False)
__mapper_args__ = {
'polymorphic_identity':ET_BRAND,
}
Im confused by this as well - are you saying that you map the same class
to B, C, and D rows? That would be unusual. It wouldn't work at all on
the persistence side as SQLAlchemy could not know which of B, C, or D you
wish for a particular BrandModel to be persisted towards.
The problem is I need to reflect something more like this:
A
/ | \
B C D
/ \
EF
Where D is not only a polymorphic child of A but also the polymorphic
parents of E F.
It seems like I have to choose, D can either be a polymorphic child or
it can be a parent - it can't be both.
Do I have any options here?
SQLAlchemy can represent inheritance hierarchies of any depth.However,
because you are assigning a single subclass to all of B, C, and D that
might be why there's an issue here, you'd need to assign a distinct
subclass of BaseModel to at least D, and then another subclass of D_Model
to handle E and F.
Preferably, you'd produce distinct classes for all six tables.
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Re: [sqlalchemy] nested inheritance / polymorphic relationships
Hello.
It seems like I have to choose, D can either be a polymorphic child or it can
be
a parent - it can't be both.
Do I have any options here?
I am almost sure you are correct. This is not possible in SA so you have to
flatten your hierarchy.
I don't know about experiences with inheritance of others on this list, but mine
in SA0.7.9 is not that good. I encountered various limitations along the way. I
guess it dependes on the complexity of the queries. We plan to get rid of it
eventually in our app. However, if I am not mistaken, SA0.8 addresses all the
quirks.
Ladislav Lenart
On 4.6.2013 07:55, Amir Elaguizy wrote:
Hi there,
I have a tree that looks like this, reflected via polymorphic inheritance:
A
/ | \
B C D
That works great, like:
class BaseModel(db.Model): # Table A in diagram
__tablename__ = entities
id = db.Column(db.BigInteger, primary_key=True, nullable=False,
server_default=func.nextval('guid_seq'))
type_id = db.Column(db.SmallInteger, db.ForeignKey(EntityTypesModel.id))
__mapper_args__ = {
'polymorphic_identity':'entity',
'polymorphic_on':type_id,
'with_polymorphic':'*'
}
class BrandModel(BaseModel): # Table B, C, D in diagram
__tablename__ = 'brands'
id = db.Column(db.BigInteger, db.ForeignKey(StufffModel.id),
primary_key=True, nullable=False)
name = db.Column(db.String, nullable=False)
__mapper_args__ = {
'polymorphic_identity':ET_BRAND,
}
The problem is I need to reflect something more like this:
A
/ | \
B C D
/ \
EF
Where D is not only a polymorphic child of A but also the polymorphic parents
of
E F.
It seems like I have to choose, D can either be a polymorphic child or it can
be
a parent - it can't be both.
Do I have any options here?
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