This is an example code.
A Document has many Comment(s)
PostComment extends Comment (with sqlalchemy polymorphic feature)
Some query returns different result between len(query.all())
and query.count()
- sqlalchemy version : 1.0.8
- mysql version : 5.6.25
See main function below.
What happened?
from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker, scoped_session
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import Table, Column, Integer, Float, Boolean, ForeignKey, String, Unicode, DateTime, Date, UniqueConstraint
from sqlalchemy.orm import relationship, backref
engine = create_engine('mysql://root:[email protected]:3306/document')
DBSession = scoped_session(sessionmaker(bind=engine))
Base = declarative_base()
Base.metadata.bind = engine
class Document(Base):
__tablename__ = 'document'
id = Column(Integer, primary_key=True)
class Comment(Base):
__tablename__ = 'comment'
id = Column(Integer, primary_key=True)
type = Column(String(50))
document_id = Column(Integer, ForeignKey('document.id'), primary_key=True)
document = relationship('Document', backref=backref('comments', lazy='dynamic'))
__mapper_args__= {
'polymorphic_identity' : 'comment',
'polymorphic_on' : type,
}
class PostComment(Comment):
__tablename__ = 'post_comment'
id = Column(Integer, ForeignKey('comment.id'), primary_key=True)
ready = Column(Boolean)
__mapper_args__= {
'polymorphic_identity' : 'post_comment',
}
def main():
Base.metadata.drop_all(engine)
Base.metadata.create_all(engine)
d1 = Document()
DBSession.add(d1)
d2 = Document()
DBSession.add(d2)
c1 = PostComment(document=d1, ready=True)
DBSession.add(c1)
c2 = PostComment(document=d1, ready=True)
DBSession.add(c2)
c3 = PostComment(document=d2, ready=True)
DBSession.add(c3)
c4 = PostComment(document=d2, ready=True)
DBSession.add(c4)
DBSession.commit()
query = d1.comments.filter(PostComment.ready==True)
print len(query.all()) # returns 2
print query.count() # returns 8
if __name__ == '__main__':
main()
Updates
http://docs.sqlalchemy.org/en/rel_1_0/orm/query.html#sqlalchemy.orm.query.Query.count
It says "Return a count of rows this Query would return.".
See Question&Answers more detail:
os 与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…