MCVE for https://stackoverflow.com/q/47065679/450517

from sqlalchemy import create_engine, Integer, String
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import sessionmaker, relationship
from sqlalchemy.schema import Column, ForeignKey

Base = declarative_base()


class User(Base):
    __tablename__ = 'user'
    ID = Column(Integer, primary_key=True)
    name = Column(String(250), nullable=False)


class Categories(Base):
    __tablename__ = 'categories'
    ID = Column(Integer, primary_key=True)
    name = Column(String(250), nullable=False)


class Item(Base):
    __tablename__ = 'item'
    ID = Column(Integer, primary_key=True)
    name = Column(String(250), nullable=False)
    description = Column(String(250))
    price = Column(String(8))
    category_ID = Column(Integer, ForeignKey('categories.ID'))
    category = relationship(Categories)
    user_ID = Column(Integer, ForeignKey('user.ID'))
    user = relationship(User)


engine = create_engine('sqlite:///:memory:', echo=True)
Base.metadata.create_all(engine)

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

i = ('Maelstrom', '2800',
     'A hammer forged for the gods themselves, '
     'Maelstrom allows its user to harness the '
     'power of lightning.',
     'Attack Speed', "guberland")

db_item = Item(
    name=i[0], price=i[1], description=i[2], category_ID=i[3],
    user_ID=i[4])

session.add(db_item)
session.commit()

# doesn't fail with sqlite !???
# but creates nonsensical data:
#
# sqlite> select * from item;
# 1|Maelstrom|A hammer forged for the gods themselves, Maelstrom allows \
#   its user to harness the power of lightning.|2800|Attack Speed|guberland
# sqlite> select * from user;
# sqlite> select * from categories;
# sqlite>
#