SQLAlchemy ORM 不适用于复合外键

Question

我正在尝试使用多个相关模型构建示例，如下所示。我们有一个与模型 C 有 1:n 关系的模型 B；然后我们有一个模型 A 与 B 有 n:1 关系，与 C 有 n:1 关系。（C 有一个 2 列主键）

我试过这段代码：

class C(db.Model):
    __tablename__ = 'C'
    key1 = Column(Integer, primary_key=True)
    key2 = Column(Integer, primary_key=True)
    attr1 = Column(Date)
    attr2 = Column(Boolean)
    related_b = Column(Integer, ForeignKey('B.spam'))


class B(db.Model):
    __tablename__ = 'B'
    spam = Column(Integer, default=1, primary_key=True)
    eggs = Column(String, default='eggs')
    null = Column(String)
    n_relation = relationship(C, foreign_keys='C.related_b')


class A(db.Model):
    __tablename__ = 'A'
    foo = Column(String, default='foo', primary_key=True)
    bar = Column(String, default='bar', primary_key=True)
    baz = Column(String, default='baz')
    rel = relationship(B, foreign_keys='A.related_b')
    related_b = Column(Integer, ForeignKey('B.spam'))
    related_c1 = Column(Integer, ForeignKey('C.key1'))
    related_c2 = Column(Integer, ForeignKey('C.key2'))
    other_rel = relationship(C, foreign_keys=(related_c1, related_c2))

只是为了得到例外：

sqlalchemy.exc.AmbiguousForeignKeysError: Could not determine join condition between parent/child tables on relationship A.other_rel - there are multiple foreign key paths linking the tables.  Specify the 'foreign_keys' argument, providing a list of those columns which should be counted as containing a foreign key reference to the parent table.

但是嘿，我已经通过了那个论点。我为该参数尝试了各种版本，通过名称指定 A 列、C 列、通过直接引用指定 C 列，似乎没有任何影响此错误。我还尝试使用单个复合外键，如下所示：

class A(db.Model):
    __tablename__ = 'A'
    foo = Column(String, default='foo', primary_key=True)
    bar = Column(String, default='bar', primary_key=True)
    baz = Column(String, default='baz')
    rel = relationship(B, foreign_keys='A.related_b')
    related_b = Column(Integer, ForeignKey('B.spam'))
    related_c1 = Column(Integer, ForeignKey('C.key1'))
    related_c2 = Column(Integer, ForeignKey('C.key2'))
    compound = ForeignKeyConstraint(('related_c1', 'related_c2'), ('C.key1', 'C.key2'))
    other_rel = relationship(C, foreign_keys=compound)

但没有任何改变。我是误会了什么还是错误？（至少，错误信息是不正确的...）

Answer 1

这里的问题是您必须在 __table_args__ 中声明 ForeignKeyConstraint，而不是在 class.

的正文中声明

换句话说，下面的代码不会将外键约束应用于子 table ...

from sqlalchemy import create_engine, Column, Integer, text, ForeignKeyConstraint, String
from sqlalchemy.orm import declarative_base, relationship

connection_url = r"mssql+pyodbc://@.\SQLEXPRESS/myDb?driver=ODBC+Driver+17+for+SQL+Server"
engine = create_engine(connection_url, echo=True,)

with engine.connect() as conn:
    for tb_name in ["tbl_child", "tbl_parent"]:
        conn.execute(text(f"DROP TABLE IF EXISTS [{tb_name}]"))

Base = declarative_base()


class Parent(Base):
    __tablename__ = "tbl_parent"
    id1 = Column(Integer, primary_key=True)
    id2 = Column(Integer, primary_key=True)
    parent_name = Column(String(50))
    children = relationship("Child", back_populates="parent")


class Child(Base):
    __tablename__ = "tbl_child"
    id = Column(Integer, primary_key=True, autoincrement=False)
    child_name = Column(String(50))
    parent_id1 = Column(Integer)
    parent_id2 = Column(Integer)
    ForeignKeyConstraint(
        ["parent_id1", "parent_id2"], ["tbl_parent.id1", "tbl_parent.id2"]
    )
    parent = relationship(
        "Parent",
        foreign_keys="[Child.parent_id1, Child.parent_id2]",
        back_populates="children",
    )


Base.metadata.create_all(engine)

""" console output:
2020-04-30 06:57:13,899 INFO sqlalchemy.engine.Engine 
CREATE TABLE tbl_parent (
    id1 INTEGER NOT NULL, 
    id2 INTEGER NOT NULL, 
    parent_name VARCHAR(50), 
    PRIMARY KEY (id1, id2)
)


2020-04-30 06:57:13,899 INFO sqlalchemy.engine.Engine ()
2020-04-30 06:57:13,900 INFO sqlalchemy.engine.Engine COMMIT
2020-04-30 06:57:13,901 INFO sqlalchemy.engine.Engine 
CREATE TABLE tbl_child (
    id INTEGER NOT NULL, 
    child_name VARCHAR(50), 
    parent_id1 INTEGER, 
    parent_id2 INTEGER, 
    PRIMARY KEY (id)
)


2020-04-30 06:57:13,901 INFO sqlalchemy.engine.Engine ()
2020-04-30 06:57:13,901 INFO sqlalchemy.engine.Engine COMMIT
"""

...但这将 ...

from sqlalchemy import create_engine, Column, Integer, text, ForeignKeyConstraint, String
from sqlalchemy.orm import declarative_base, relationship

connection_url = r"mssql+pyodbc://@.\SQLEXPRESS/myDb?driver=ODBC+Driver+17+for+SQL+Server"
engine = create_engine(connection_url, echo=True,)

with engine.connect() as conn:
    for tb_name in ["tbl_child", "tbl_parent"]:
        conn.execute(text(f"DROP TABLE IF EXISTS [{tb_name}]"))

Base = declarative_base()


class Parent(Base):
    __tablename__ = "tbl_parent"
    id1 = Column(Integer, primary_key=True)
    id2 = Column(Integer, primary_key=True)
    parent_name = Column(String(50))
    children = relationship("Child", back_populates="parent")


class Child(Base):
    __tablename__ = "tbl_child"
    __table_args__ = (
        ForeignKeyConstraint(
            ["parent_id1", "parent_id2"], ["tbl_parent.id1", "tbl_parent.id2"]
        ),
    )
    id = Column(Integer, primary_key=True, autoincrement=False)
    child_name = Column(String(50))
    parent_id1 = Column(Integer)
    parent_id2 = Column(Integer)

    parent = relationship(
        "Parent",
        foreign_keys="[Child.parent_id1, Child.parent_id2]",
        back_populates="children",
    )


Base.metadata.create_all(engine)

""" console output:
CREATE TABLE tbl_parent (
    id1 INTEGER NOT NULL, 
    id2 INTEGER NOT NULL, 
    parent_name VARCHAR(50) NULL, 
    PRIMARY KEY (id1, id2)
)


2020-04-30 07:52:43,771 INFO sqlalchemy.engine.Engine ()
2020-04-30 07:52:43,776 INFO sqlalchemy.engine.Engine COMMIT
2020-04-30 07:52:43,778 INFO sqlalchemy.engine.Engine 
CREATE TABLE tbl_child (
    id INTEGER NOT NULL, 
    child_name VARCHAR(50) NULL, 
    parent_id1 INTEGER NULL, 
    parent_id2 INTEGER NULL, 
    PRIMARY KEY (id), 
    FOREIGN KEY(parent_id1, parent_id2) REFERENCES tbl_parent (id1, id2)
)


2020-04-30 07:52:43,778 INFO sqlalchemy.engine.Engine ()
2020-04-30 07:52:43,802 INFO sqlalchemy.engine.Engine COMMIT
"""

参考：

https://docs.sqlalchemy.org/en/14/orm/declarative_tables.html#orm-declarative-table-configuration

SQLAlchemy ORM 不适用于复合外键

SQLAlchemy ORM not working with composite foreign keys

python

orm

sqlalchemy

foreign-keys

relationship