多对多关系中的相关记录数 [英] Count of related records in many-to-many relationship

问题描述

我正在尝试构建一个类方法，它返回与项目关联的成员数.我试过了:

I am trying to buit a classmethod, which returns number of members associated with a project. I tried:

# 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！

推荐答案

或者使用像下面这样的简单属性，它将使用实例的当前会话来获取孩子的数量

Either use a simple property like below, which will use the current session of the instance to get count of children

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

或使用混合属性:

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，您可以在查询中使用此表达式(例如，filters 和 order_by):

... in which case you can use this expression in your query (filters and order_by, for example):

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)

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