Python 多对多关系中相关记录的计数

我正在尝试构建一个classmethod，它返回与项目关联的成员数。我试过：

# method of class Project
@classmethod
def member_count(cls, project_id):
   return Session.query(ProjectMember).\
            filter(ProjectMember.project_id==project_id).count()

多对多关系定义为：

class Member(Base):
    __tablename__ = 'member'
    id = Column(Integer, primary_key=True)
    login = Column(String(50), unique=True, nullable=False)
    project_list = relationship("ProjectMember", backref="member")

class Project(Base):
    __tablename__ = 'project'
    id = Column(Integer, primary_key=True)
    name = Column(String(100), unique=True, nullable=False)

class ProjectMember(Base):
    __tablename__ = 'project_member'
    project_id = Column(Integer, ForeignKey("project.id"), nullable=False, primary_key=True)
    member_id = Column(Integer, ForeignKey("member.id"), nullable=False, primary_key=True)
    project = relationship("Project", backref = "project_member")
    is_pm = Column(Boolean, default = True, nullable = False)
    UniqueConstraint('project_id', 'member_id')

---

谢谢

或者使用下面这样的简单属性，该属性将使用实例的当前会话来获取子级的计数

class Project(...):
    # ...
    @property
    def member_count_simple(self):
       return object_session(self).query(ProjectMember).with_parent(self).count()


print(my_proj.member_count_simple) # @note: will issue an SQL statement

或使用：

。。。在这种情况下，您可以在查询中使用此表达式（

filter

s和

order\u by

），例如：

---

或者使用下面这样的简单属性，该属性将使用实例的当前会话来获取子级的计数

class Project(...):
    # ...
    @property
    def member_count_simple(self):
       return object_session(self).query(ProjectMember).with_parent(self).count()


print(my_proj.member_count_simple) # @note: will issue an SQL statement

或使用：

。。。在这种情况下，您可以在查询中使用此表达式（

filter

s和

order\u by

），例如：

---

当您尝试执行该方法时会发生什么？您是否遇到错误？当您尝试执行该方法时会发生什么情况？您是否收到错误？van，对象会话（self）来自哪里？来自哪里？van，对象会话（self）来自哪里？来自哪里？

qry = (session.query(Project, Project.member_count)
        .order_by(Project.member_count.desc())
        )
# @note: it returns tuples (Project, member_count)
for (proj, cnt_mem) in qry.all():
    print(proj.name, cnt_mem)