使用PostgreSQL数组存储多对多关系

Question

假设我们有一个 PostgreSQL 数据库，其中包含两个表 A、B。

表 A 列：id、name 表 B 列：id、name、array_a

表 B 中的列 array_a 包含来自表 A 的可变长度 id 数组。在 SQLAlchemy 中，我们有两个类对这些表进行建模，例如类 A 和 B。

以下方法可以很好地获取对象 B 中引用的所有对象 A：

session.query(A).join(B, A.id == func.any(B.array_a)).filter(B.id == <id>).all()

我们如何在 B 中创建一个引用数组对应的对象 A 的关系？使用上面的func.any 尝试了列比较器，但它抱怨ANY(array_a) 不是模型中的列。如上所述指定primaryjoin条件似乎也没有削减它。

Answer 1

这个反模式叫做"Jaywalking"；而 PostgreSQL 强大的类型系统让它非常诱人。你应该使用另一个表：

CREATE TABLE table_a (
    id SERIAL PRIMARY KEY,
    name VARCHAR
);

CREATE TABLE table_b (
    id SERIAL PRIMARY KEY,
    name VARCHAR
);

CREATE TABLE a_b (
    a_id INTEGER PRIMARY KEY REFERENCES table_a(id),
    b_id INTEGER PRIMARY KEY REFERENCES table_b(id)
)

已映射：

from sqlalchemy import *
from sqlalchemy.dialects import postgresql
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import *

Base = declarative_base()

a_b_table = Table("a_b", Base.metadata,
    Column("a_id", Integer, ForeignKey("table_a.id"), primary_key=True),
    Column("b_id", Integer, ForeignKey("table_b.id"), primary_key=True))

class A(Base):
    __tablename__ = "table_a"
    id = Column(Integer, primary_key=True)
    name = Column(String)

class B(Base):
    __tablename__ = "table_b"
    id = Column(Integer, primary_key=True)
    name = Column(String)
    a_set = relationship(A, secondary=a_b_table, backref="b_set")

示例：

>>> print Query(A).filter(A.b_set.any(B.name == "foo"))
SELECT table_a.id AS table_a_id, table_a.name AS table_a_name 
FROM table_a 
WHERE EXISTS (SELECT 1 
FROM a_b, table_b 
WHERE table_a.id = a_b.a_id AND table_b.id = a_b.b_id AND table_b.name = :name_1)

如果您被ARRAY 列所困扰，最好的办法是使用“看起来”像正确关联表的备用可选选项。

from sqlalchemy import *
from sqlalchemy.dialects import postgresql
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import *

Base = declarative_base()


class A(Base):
    __tablename__ = "table_a"
    id = Column(Integer, primary_key=True)
    name = Column(String)

class B(Base):
    __tablename__ = "table_b"
    id = Column(Integer, primary_key=True)
    name = Column(String)
    array_a = Column(postgresql.ARRAY(Integer))

a_b_selectable = select([func.unnest(B.array_a).label("a_id"),
                         B.id.label("b_id")]).alias()

A.b_set = relationship(B, secondary=a_b_selectable,
                          primaryjoin=A.id == a_b_selectable.c.a_id,
                          secondaryjoin=a_b_selectable.c.b_id == B.id,
                          viewonly=True,)

B.a_set = relationship(A, secondary=a_b_selectable,
                          primaryjoin=A.id == a_b_selectable.c.a_id,
                          secondaryjoin=a_b_selectable.c.b_id == B.id,
                          viewonly=True)

给你：

>>> print Query(A).filter(A.b_set.any(B.name == "foo"))
SELECT table_a.id AS table_a_id, table_a.name AS table_a_name 
FROM table_a 
WHERE EXISTS (SELECT 1 
FROM (SELECT unnest(table_b.array_a) AS a_id, table_b.id AS b_id 
FROM table_b) AS anon_1, table_b 
WHERE table_a.id = anon_1.a_id AND anon_1.b_id = table_b.id AND table_b.name = :name_1)

显然，由于那里没有真正的桌子，viewonly=True 是必要的，如果您避免乱穿马路，您将无法获得良好的、动态的客观善良。

Answer 2

或者你可以像下面这样明确地加入：

class A(Base):
    __tablename__ = "table_a"
    id = Column(Integer, primary_key=True)
    name = Column(String)

class B(Base):
    __tablename__ = "table_b"
    id = Column(Integer, primary_key=True)
    name = Column(String)
    array_a = Column(postgresql.ARRAY(Integer))

    a_ids= relationship('A',primaryjoin='A.id == any_(foreign(B.array_a))',uselist=True)