I have a base Dataset class that I'd like to dynamically inherit from. When
a user uploads a new dataset, I create a new table as shown below, and
populate it with data. My two most common use-cases:
1. Retrieving entire dataset. I want to return dataset_x where x is an
integer that corresponds to experiment_id
2. Searching through all datasets. I query against the parent datasets table
and apply filters as needed.
The issue is that when I need to retrieve a dataset, I need to run through
my create_dataset function once more. This isn't such a big deal for use
case #1, I just do create_dataset(x). However, for case #2, I need to loop
through the whole set of child tables. My question is: is there a way
around this? Additionally, please tell me if what I'm doing is basically
pointless.. my child classes do not add additional attributes. I chose this
route because it seemed to make sense based on my two use cases above. Each
dataset is on the order of 10,000 rows, and I expect to have on the order
of 10,000 datasets. Should I just stick to having an additional column for
experiment_id and just filter against then in case #1?
I should note that I am using Flask-SQLAlchemy but I think this maps well
to the pure SQLAlchemy case.. Please tell me if I'm off base on that
assumption based on the below code. My parent class:
class Dataset(db.Model):
__tablename__ = 'datasets'
id = Column(db.Integer, primary_key=True)
foo = Column(db.Integer)
bar = Column(db.String(20))
experiment_id = Column(db.Integer)
__mapper_args__ = {
'polymorphic_identity': 'dataset',
'polymorphic_on': experiment_id
}
How I create new child classes dynamically:
def create_dataset(experiment_id):
dataset_name = 'dataset_{}'.format(experiment_id)
props = {
'__tablename__': dataset_name,
'__mapper_args__': {'polymorphic_identity': experiment_id},
'id': db.Column(db.Integer, db.ForeignKey('datasets.id'),
primary_key=True)
}
return type(
dataset_name,
(Dataset,),
props
)
The error that I run up against if I neglect running my dataset creation
function:
AssertionError: No such polymorphic_identity 1 is defined
and the larger stack trace:
File "/env/lib/python3.5/site-packages/sqlalchemy/orm/loading.py", line 86
, in instances
util.raise_from_cause(err)
File "/env/lib/python3.5/site-packages/sqlalchemy/util/compat.py", line
200, in raise_from_cause
reraise(type(exception), exception, tb=exc_tb, cause=cause)
File "/env/lib/python3.5/site-packages/sqlalchemy/util/compat.py", line
184, in reraise
raise value
File "/env/lib/python3.5/site-packages/sqlalchemy/orm/loading.py", line 71
, in instances
rows = [proc(row) for row in fetch]
File "/env/lib/python3.5/site-packages/sqlalchemy/orm/loading.py", line 71
, in <listcomp>
rows = [proc(row) for row in fetch]
File "/env/lib/python3.5/site-packages/sqlalchemy/orm/loading.py", line
594, in polymorphic_instance
_instance = polymorphic_instances[discriminator]
File "/env/lib/python3.5/site-packages/sqlalchemy/util/_collections.py",
line 728, in __missing__
self[key] = val = self.creator(key)
File "/env/lib/python3.5/site-packages/sqlalchemy/orm/loading.py", line
578, in configure_subclass_mapper
discriminator)
AssertionError: No such polymorphic_identity 1 is defined
Thank you for the wonderful ORM!
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