SQLAlchemy ORM not working with composite foreign keys

Question

I\'m trying to build an example with several related models, like the following. We have a model B with a 1:n relation with a model C; then we have a model A with a n:1 relation

Answer 1

The issue here is that you have to declare the ForeignKeyConstraint in __table_args__, not in the body of the class.

In other words, the following code will NOT apply the foreign key constraint to the child table ...

from sqlalchemy import (
    create_engine,
    Column,
    Integer,
    text,
    ForeignKeyConstraint,
    String,
)
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import relationship

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

    def __repr__(self):
        return f"<Parent(id1={self.id1}, id2={self.id2}), parent_name='{self.parent_name}'>"


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",
    )

    def __repr__(self):
        return f"<Child(id={self.id}, child_name={self.child_name})>"


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
"""

... but this will ...

from sqlalchemy import (
    create_engine,
    Column,
    Integer,
    text,
    ForeignKeyConstraint,
    String,
)
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import relationship

connection_uri = (
    r"mssql+pyodbc://@.\SQLEXPRESS/myDb?driver=ODBC+Driver+17+for+SQL+Server"
)
# connection_uri = "sqlite:///:memory:"
engine = create_engine(connection_uri, 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")

    def __repr__(self):
        return f"<Parent(id1={self.id1}, id2={self.id2}), parent_name='{self.parent_name}'>"


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",
    )

    def __repr__(self):
        return f"<Child(id={self.id}, child_name={self.child_name})>"


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
"""

Reference:

https://docs.sqlalchemy.org/en/13/orm/extensions/declarative/table_config.html#table-configuration