ForeingKey experiments on SQLAlchemy. It seems following 2 python codes are equivalent. According to my experiments, if you want to have `relationship`, you need to use explicitly any `foreign` by `ForeignKey` schema or `foreign` annotation. Anyway you can get the power of `relationship` with or without the foreign key constraints.

db.sqlite3

CREATE TABLE addresses (
	id INTEGER NOT NULL, 
	email VARCHAR NOT NULL, 
	owner_id INTEGER, 
	PRIMARY KEY (id)
);
CREATE TABLE users (
	id INTEGER NOT NULL, 
	name VARCHAR NOT NULL, 
	PRIMARY KEY (id)
);

with_fkey_annotation_in_join_condition.py

from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.schema import Column, ForeignKey
from sqlalchemy.types import Integer, String
from sqlalchemy.orm import relation
from sqlalchemy.engine import create_engine
from sqlalchemy.orm.session import sessionmaker
from sqlalchemy.orm import foreign

Base = declarative_base()


class User(Base):
    __tablename__ = 'users'

    id = Column(Integer, primary_key=True)
    name = Column(String, nullable=False)
    addresses = relation('Address', primaryjoin='foreign(Address.owner_id) == User.id')


class Address(Base):
    __tablename__ = 'addresses'

    id = Column(Integer, primary_key=True)
    email = Column(String, nullable=False)
    owner_id = Column(Integer)

with_fkey_schema.py

from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.schema import Column, ForeignKey
from sqlalchemy.types import Integer, String
from sqlalchemy.orm import relation
from sqlalchemy.engine import create_engine
from sqlalchemy.orm.session import sessionmaker

Base = declarative_base()


class User(Base):
    __tablename__ = 'users'

    id = Column(Integer, primary_key=True)
    name = Column(String, nullable=False)
    addresses = relation('Address')


class Address(Base):
    __tablename__ = 'addresses'

    id = Column(Integer, primary_key=True)
    email = Column(String, nullable=False)
    owner_id = Column(Integer, ForeignKey('users.id'))