sqlalchemy.exc.IntegrityError: (sqlite3.IntegrityError) NOT NULL constraint failed

Question

i recentry started to learn programing, and i run into issue. Im using flask wtffroms and just want to create a delete route, every time i try to delete my post i geting this error

sqlalchemy.exc.IntegrityError: (sqlite3.IntegrityError) NOT NULL constraint failed: prekes_nuotraukos.preke_id
[SQL: UPDATE prekes_nuotraukos SET preke_id=? WHERE prekes_nuotraukos.id = ?]
[parameters: ((None, 20), (None, 21), (None, 22), (None, 23))]
(Background on this error at: http://sqlalche.me/e/13/gkpj)

i think the problem is that i use one wtfform variables for 2 sqlalchemy tables and it cant find prekes_nuotraukos.id because its in different table or i missing something. This is my model.db

class Preke(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    pavadinimas = db.Column(db.String(30), nullable=False)
    apibudinimas = db.Column(db.String(80), nullable=False)
    creation_time = db.Column(db.DateTime, default=datetime.utcnow)
    user_id = db.Column(db.Integer, db.ForeignKey("user.id"), nullable=False)
    items = db.relationship("PrekesNuotraukos", backref="PrekesFT", lazy=True, uselist=True)

    def __repr__(self):
        return f"Post('{self.pavadinimas}', '{self.apibudinimas}', {self.creation_time})"

class PrekesNuotraukos(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    prekes_nuotraukos = db.Column(db.String(20), nullable=False)
    creation_time = db.Column(db.DateTime, default=datetime.utcnow)
    preke_id = db.Column(db.Integer, db.ForeignKey("preke.id"), nullable=False)

    def __repr__(self):
        return f"Post('{self.creation_time}, {self.prekes_nuotraukos})"

and code for front end portion

<button type="button" class="btn btn-secondary" data-dismiss="modal">Close</button>
          <form action="{{url_for('delete_Preke', p_id=preke.id)}}" method="post">
              <input class="btn btn-danger" type="submit" value="Delete">
          </form>

code for routing:

@app.route("/app_main_page/preke/<int:p_id>/delete",  methods=["POST"])
@login_required
def delete_Preke(p_id):
    preke = Preke.query.get_or_404(p_id)
    if preke.author != current_user:
        abort(403)
    db.session.delete(preke)
    db.session.commit()
    flash("Jusu preke buvo sekmingai istrinta", "success")
    return redirect(url_for(visos_prekes))

Answer 1

This example code (pure SQLAlchemy, not Flask-SQLAlchemy I'm afraid, but they are very similar) shows how adding a delete cascade removes the error on deletion.

import sqlalchemy as sa
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import orm


Base = declarative_base()


class Preke(Base):
    __tablename__ = 'preke'
    id = sa.Column(sa.Integer, primary_key=True)
    pavadinimas = sa.Column(sa.String(30), nullable=False)
    # If we remove the "cascade='...'" from the next line, we get the error.
    items = sa.orm.relationship("PrekesNuotraukos", backref="PrekesFT", lazy=True, uselist=True, cascade='all, delete-orphan')


class PrekesNuotraukos(Base):
    __tablename__ = 'preke_nuotraukos'
    id = sa.Column(sa.Integer, primary_key=True)
    preke_id = sa.Column(sa.Integer, sa.ForeignKey("preke.id"), nullable=False)


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

Session = orm.sessionmaker(engine)
session = Session()

p = Preke(pavadinimas='A')
session.add(p)
pk1 = PrekesNuotraukos(PrekesFT=p)
pk2 = PrekesNuotraukos(PrekesFT=p)
session.add_all([pk1, pk2])
session.commit()
session.close()

session = Session()
p = session.query(Preke).first()
session.delete(p)
session.commit()
session.close()