There's examples of custom SQL expressions at:
http://docs.sqlalchemy.org/en/latest/orm/mapping_api.html#sqlalchemy.orm.mapper.params.polymorphic_on
These examples don't currently illustrate a subquery (which would be a
good idea), but the idea is you make a scalar correlated subquery (a
Core example is at
http://docs.sqlalchemy.org/en/latest/core/tutorial.html#correlated-subqueries)
and you can stick that right into polymorphic_on, or send it as a
column_property.
class Publication(db.Model):
__tablename__ = 'publication'
id = db.Column(db.Integer, primary_key=True)
series_id = db.Column(db.Integer, db.ForeignKey('series.id'),
nullable=False)
series = db.relationship(Series, backref='publications')
__mapper_args__ = {
'polymorphic_on': select([series.type]).where(series_id ==
Series.id).as_scalar(),
'polymorphic_identity': None
}
On 01/27/2017 06:20 PM, Shane Carey wrote:
Hi, I want to use a discriminator based upon a column of a related table.
I found the relevant place in the
docs
http://docs.sqlalchemy.org/en/latest/orm/inheritance.html#joined-table-inheritance
"The discriminator column is only needed if polymorphic loading is
desired, as is usually the case. It is not strictly necessary that it be
present directly on the base mapped table, and can instead be defined on
a derived select statement that’s used when the class is queried;
however, this is a much more sophisticated configuration scenario"
However this is not working for me, additionally I found this
stackoverflow question which also encounters this problem to a
tee.
http://stackoverflow.com/questions/40862634/sqlalchemy-orm-polymorphic-on-from-relationship
What is the correct method to have a discriminator based on a more
complex select statement? Thank you!
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http://www.sqlalchemy.org/
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