NoForeignKeysError: Could not determine join condition ... there are no foreign keys linking these tables

orm, python, sqlalchemy

Solution

Your `Post` model has no `thread` reference. Add a column to `Post` referencing the `Thread` a post belongs to:

class Post(Base):
    __tablename__ = "post"
    id = Column(Integer, primary_key=True)
    name = Column(String)
    body = Column(String)
    author = Column(Integer, ForeignKey("user.id"))
    thread_id = Column(Integer, ForeignKey('thread.id'))

We can't use the name `thread` because that's what the `Post.replies` relationship will add to retrieved `Thread` instances.

This is a One to Many relationship as documented in the SQLAlchemy Relationship Configuration documentation.

Problem

Im using sqlalchemy to design a forum style website. I started knocking out the design but everytime I try to test it with a few inserts, it dumps a brick; ``` NoForeignKeysError: Could not determine join condition between parent/child tables on relationship Thread.replies - there are no foreign keys linking these tables. Ensure that referencing columns are associated with a ForeignKey or ForeignKeyConstraint, or specify a 'primaryjoin' expression. ``` Here are my "models" ``` from sqlalchemy import Integer, Column, String, create_engine, ForeignKey from sqlalchemy.orm import relationship, sessionmaker, backref from .database import Base # declarative base instance class User(Base): __tablename__ = "user" id = Column(Integer, primary_key=True) username = Column(String, unique=True) email = Column(String, unique=True) threads = relationship("Thread", backref="user") posts = relationship("Post", backref="user") class Post(Base): __tablename__ = "post" id = Column(Integer, primary_key=True) name = Column(String) body = Column(String) author = Column(Integer, ForeignKey("user.id")) class Thread(Base): __tablename__ = "thread" id = Column(Integer, primary_key=True) name = Column(String) desc = Column(String) replies = relationship("Post", backref="thread") author_id = Column(Integer, ForeignKey("user.id")) board_id = Column(Integer, ForeignKey("board.id")) class Board(Base): __tablename__ = "board" id = Column(Integer, primary_key=True) name = Column(String) desc = Column(String) threads = relationship("Thread", backref="board") category_id = Column(Integer, ForeignKey("category.id")) class Category(Base): __tablename__ = "category" id = Column(Integer, primary_key=True) name = Column(String) desc = Column(String) threads = relationship("Board", backref="category") engine = create_engine('sqlite:///:memory:', echo=True) Base.metadata.create_all(engine) session_factory = sessionmaker(bind=engine) session = session_factory() ```

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