well, i have that feeling too. it's a basic NxN relationship between the
same instance type. the anti-pattern might be having a polymorphic model
to do that. i know sqlalchemy can handle this issue and provide me
constant results, but by hand it can give me trouble ... should I remove
the polymorphic association for logic' sake?
On 01/17/2014 03:24 PM, Michael Bayer wrote:
On Jan 17, 2014, at 12:05 PM, Richard Gerd Kuesters
<[email protected] <mailto:[email protected]>> wrote:
hi all,
I'm having a tricky situation while modeling a database here. first,
i'll just put the code of my two classes / models:
*
**class Resource(Base):**
**
** resource_id = Column(Integer, Sequence, primary_key=True)**
** resource_type_id = Column(Integer, ForeignKey(SomeClass.id),
nullable=False, index=True)**
** p_discriminator = Column(Integer)**
**
** __mapper_args__ = dict(**
** polymorphic_on=p_discriminator,**
** polymorphic_identity=1**
** )**
**
**class ResourceRelationship(Resource):**
**
** resource_id = Column(Integer, ForeignKey(Resource.resource_id),
primary_key=True, index=True)**
** left_resource_id = Column(Integer,
ForeignKey(Resource.resource_id), nullable=True, index=True)**
** right_resource_id = Column(Integer,
ForeignKey(Resource.resource_id), nullable=True, index=True)**
**
** __mapper_args__ = dict(**
** polymorphic_identity=2**
** inherit_condition=(resource_id == Resource.resource_id)**
** )*
ok, nothing fancy but, I would really like to create a unique
constraint on ResourceRelationship, using left_resouce_id,
right_resource_id AND the inherited resource_type_id.
the problem is: i can do that only using concrete inheritance, afaik.
i would not like to do that because i really like the way polymorphic
models work -- and the most important reason: this database is shared
with other applications that would not rely on sqlalchemy to do the
magic of maintain the data integrity.
you’d need to add resource_type_id to ResourceRelationship as well.
The SQLAlchemy mapping will maintain the same value for both columns
provided you map them under the same attribute name.
i have a sneaky feeling there’s some relational design antipattern
occurring here but I’m not adept enough to determine specifically what
that might be.
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