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@edelooff

edelooff/column_flag.py

Last active May 12, 2020
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SQLAlchemy columns with a hybrid property boolean flag to check for presence (non-nullness)
from sqlalchemy import (
Column,
Integer,
DateTime,
Text,
create_engine,
func)
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.ext.hybrid import hybrid_property
from sqlalchemy.inspection import inspect
from sqlalchemy.orm import Session
Base = declarative_base()
def column_flag(column, **options):
def target_key(instance):
target = inspect(instance).mapper.get_property_by_column(column)
return target.key
def fget(self):
return getattr(self, target_key(self)) is not None
def fset(self, value, default=options.get('default')):
if not isinstance(value, bool):
raise TypeError('Flag only accepts boolean values')
return setattr(self, target_key(self), default if value else None)
def expr(cls):
return column.isnot(None)
return hybrid_property(
fget=fget,
fset=fset if 'default' in options else None,
expr=expr)
class Article(Base):
__tablename__ = 'article'
id = Column(Integer, primary_key=True)
content = Column(Text)
published_at = Column('publication_date', DateTime)
is_published = column_flag(published_at, default=func.now())
def main():
engine = create_engine('sqlite://', echo=True)
Base.metadata.create_all(bind=engine)
session = Session(bind=engine)
art1 = Article(content='First post', published_at=func.now())
art2 = Article(content='Tentative content')
session.add_all([art1, art2])
session.flush()
count_total = session.query(Article).count()
count_published = session.query(Article)\
.filter(Article.is_published)\
.count()
print(f'Articles published out of total: {count_published}/{count_total}')
assert art1.is_published
assert not art2.is_published
# Publish article 2
art2.is_published = True
session.flush()
print(f'Article 2 was published at {art2.published_at}')
# Unpublish article 2
art2.is_published = False
session.flush()
print(f'Article 2 publication date: {art2.published_at}')
if __name__ == '__main__':
main()
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