On Wed, Nov 7, 2018 at 12:33 PM <[email protected]> wrote:
>
> I have the following two models
>
> class Dataset(db.Model):
> __tablename__ = 'datasets'
>
> id = db.Column(db.Integer, primary_key=True)
> creation_datetime = db.Column(db.DateTime(timezone=False), nullable=False)
>
> sample_id = db.Column(db.Integer, db.ForeignKey('samples.id'),
> nullable=False, index=True)
> sample = db.relationship('Sample', uselist=False, innerjoin=True)
>
>
> class Sample(db.Model):
> __tablename__ = 'samples'
>
>
> id = db.Column(db.Integer, primary_key=True)
>
> datasets = db.relationship('Dataset')
>
> num_datasets = column_property(
> select([func.count(Dataset.id)]).where(Dataset.sample_id ==
> id).correlate_except(Dataset)
> )
>
>
>
> A sample has many datasets. I would like to add a property "is_latest" to
> Dataset, probably as a column_property, which is true if the dataset has the
> latest creation_datetime of the datasets associated with its sample. In other
> words, if a sample has three datasets, the dataset whose creation_datetime is
> largest should have is_latest=True and the other two should have
> is_latest=False
>
> If ordering by creation_datetime isn't possible/easy, ordering by primary key
> is also acceptable. How can I construct this column_property?
>
> One of the issues I've encountered while trying to make this work is that
> there's a circular dependency between the two models. Since the Sample model
> uses Dataset in its num_datasets property, the Dataset class is currently
> defined first. But this means that it can't refer to Sample in its own
> properties
do you need help with the structure of the SQL for is_latest or you're
just struggling with the definitions for the two column_property
objects in that they each require the other class? for the latter,
you need to use a __declare_last__ :
class MyClass(Base):
# ...
@classmethod
def __declare_last__(cls):
cls.is_latest = column_property(....)
>
> Thanks for the help
>
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> --
> SQLAlchemy -
> The Python SQL Toolkit and Object Relational Mapper
>
> http://www.sqlalchemy.org/
>
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--
SQLAlchemy -
The Python SQL Toolkit and Object Relational Mapper
http://www.sqlalchemy.org/
To post example code, please provide an MCVE: Minimal, Complete, and Verifiable
Example. See http://stackoverflow.com/help/mcve for a full description.
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