sqlalchemy 中多列的唯一约束

Question

我想对表施加约束以限制表中值的输入。 我想确保任何值的组合只能出现一次。

即（1,2）和（2,1）不能在同一个表中。

例如，我有一个包含两列（c1 和 c2）的表：
必须遵循以下几点。

C1        C2
 1         2         OK 
 2         1         NOT OK
 3         1         OK
 1         2         NOT OK
 1         4         OK
 1         3         NOT OK

有没有办法在 SQLALchemy 中做到这一点？

我使用了 UNIQUE(c1, c2) 但这只是说 (1,2) 和 (1,2) 不能在同一个表中但如前所述我也想包括 (2,1) 不能在表。

谢谢

Answer 1

可能最简单的解决方案是为c1 < c2 （或c1 <= c2如果允许它们相同）添加检查约束，以便 (c1, c2) 将始终处于“升序”：

import sqlalchemy as sa

connection_uri = (
    "mssql+pyodbc://@localhost:49242/myDb?driver=ODBC+Driver+17+for+SQL+Server"
)
engine = sa.create_engine(connection_uri)
Base = declarative_base()


class So64232358(Base):
    __tablename__ = "so64232358"
    id = sa.Column(sa.Integer, primary_key=True, autoincrement=True)
    c1 = sa.Column(sa.Integer, nullable=False)
    c2 = sa.Column(sa.Integer, nullable=False)
    comment = sa.Column(sa.String(50))
    sa.CheckConstraint(c1 < c2)
    sa.UniqueConstraint(c1, c2)


Base.metadata.drop_all(engine, checkfirst=True)
Base.metadata.create_all(engine)
"""SQL rendered:
CREATE TABLE so64232358 (
    id INTEGER NOT NULL IDENTITY, 
    c1 INTEGER NOT NULL, 
    c2 INTEGER NOT NULL, 
    comment VARCHAR(50) NULL, 
    PRIMARY KEY (id), 
    CHECK (c1 < c2), 
    UNIQUE (c1, c2)
)
"""

Session = sessionmaker(bind=engine)
session = Session()

obj = So64232358(c1=2, c2=1, comment="no es bueno")
session.add(obj)
try:
    session.commit()
except sa.exc.IntegrityError as ie:
    print(ie)
    """console output:
    (pyodbc.IntegrityError) ('23000', '[23000] [Microsoft]
    [ODBC Driver 17 for SQL Server][SQL Server]The INSERT statement conflicted
    with the CHECK constraint "CK__so64232358__429B0397". The conflict
    occurred in database "myDb", table "dbo.so64232358".
    (547) (SQLExecDirectW);
    [23000] [Microsoft][ODBC Driver 17 for SQL Server][SQL Server]
    The statement has been terminated. (3621)')
    """
    session.rollback()
obj = So64232358(c1=1, c2=2, comment="bueno")
session.add(obj)
session.commit()  # no error
obj = So64232358(c1=1, c2=2, comment="duplicado")
session.add(obj)
try:
    session.commit()
except sa.exc.IntegrityError as ie:
    print(ie)
    """console output:
    (pyodbc.IntegrityError) ('23000', "[23000] [Microsoft]
    [ODBC Driver 17 for SQL Server][SQL Server]Violation of UNIQUE KEY
    constraint 'UQ__so642323__E13250592117193A'. Cannot insert duplicate key
    in object 'dbo.so64232358'. The duplicate key value is (1, 2).
    (2627)(SQLExecDirectW);
    [23000] [Microsoft][ODBC Driver 17 for SQL Server][SQL Server]
    The statement has been terminated. (3621)")
    """
    session.rollback()