多对多关系中的相关记录计数

Question

我正在尝试创建一个类方法，这个方法可以返回与某个项目相关的成员数量。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()

多对多的关系是这样定义的：

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！

Answer 1

你可以使用下面这样一个简单的属性，这样就能利用当前实例的会话来获取子项的数量。

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

或者你可以使用Hybrid Attribute：

class Project(Base):
    # ...

    @hybrid_property
    def member_count(self):
        return object_session(self).query(ProjectMember).with_parent(self).count()

    @member_count.expression
    def _member_count_expression(cls):
        q = select([func.count(ProjectMember.project_id)]).\
                where(ProjectMember.project_id == cls.id).\
                label("member_count")
        return q

... 在这种情况下，你可以在查询中使用这个表达式（比如在filter和order_by中）：

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)