SQLAlchemy-Utils聚合属性：如何在聚合之前应用过滤器来创建聚合字段？

Question

from sqlalchemy_utils import aggregated


class Thread(Base):
    __tablename__ = 'thread'
    id = sa.Column(sa.Integer, primary_key=True)
    name = sa.Column(sa.Unicode(255))

    @aggregated('comments', sa.Column(sa.Integer))
    def comment_count(self):
        return sa.func.count('1')

    comments = sa.orm.relationship(
        'Comment',
        backref='thread'
    )


class Comment(Base):
    __tablename__ = 'comment'
    id = sa.Column(sa.Integer, primary_key=True)
    content = sa.Column(sa.UnicodeText)
    thread_id = sa.Column(sa.Integer, sa.ForeignKey(Thread.id))
    active = sa.Column(sa.Boolean)

我希望

comment_count

字段仅统计活跃评论，而不是所有评论。可以吗？

该示例基于文档：https://sqlalchemy-utils.readthedocs.io/en/latest/aggregates.html

Answer 1

您需要 declerative_base 和 sa 才能执行此操作，您可以在此处找到有关它们的更多信息。

from sqlalchemy_utils import aggregated
import sqlalchemy as sa
from sqlalchemy.ext.declarative import declarative_base

Base = declarative_base()

class Thread(Base):
    __tablename__ = 'thread'
    id = sa.Column(sa.Integer, primary_key=True)
    name = sa.Column(sa.Unicode(255))

    @aggregated('comments', sa.Column(sa.Integer))
    def comment_count(self):
        return sa.func.count(sa.case([(Comment.active == True, 1)]))

    comments = sa.orm.relationship(
        'Comment',
        backref='thread'
    )

class Comment(Base):
    __tablename__ = 'comment'
    id = sa.Column(sa.Integer, primary_key=True)
    content = sa.Column(sa.UnicodeText)
    thread_id = sa.Column(sa.Integer, sa.ForeignKey(Thread.id))
    active = sa.Column(sa.Boolean)

1- sa.case([(Comment.active == True, 1)]) 创建一个 case 语句，如果评论处于活动状态，则返回 1，否则返回 NULL。

然后，

2-sa.func.count 仅计算 case 语句返回 1 的行，从而有效地仅计算活动评论。 这应该为您提供 comment_count 作为与每个线程关联的活动评论的数量。