Flask sqlalchemy returns integretyerror when I am trying to delete a row.

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Python3.6

flask

Hello, I have a small webapp using flask and sqlAlchemy.

Im currently stuck at one problem. The problem is that I can not remove entries from the database using sqlAlchemy. The error wich gets returned:

sqlalchemy.exc.IntegrityError: (sqlite3.IntegrityError) NOT NULL constraint failed: attachment.share_id
[SQL: UPDATE attachment SET share_id=? WHERE attachment."ID" = ?]
[parameters: (None, 1)]
(Background on this error at: http://sqlalche.me/e/gkpj)

What my codes looks like (the sql module):

class Share(db.Model):
    ID = db.Column(db.Integer, primary_key=True)
    id = db.synonym("ID")
    uuid = db.Column(db.String(36), unique=True,
                     nullable=False, default=get_unique_id)
    title = db.Column(db.String(100), nullable=True)
    content = db.Column(db.Text, nullable=False)
    date = db.Column(db.DateTime, nullable=False, default=datetime.utcnow)
    user_id = db.Column(db.Integer, db.ForeignKey('user.ID'), nullable=False)
    attachments = db.relationship(
        "Attachment", backref="share", lazy=True)
    comments = db.relationship(
        "Comment", backref="share", order_by="desc(webapp.models.Comment.date)", lazy=True)

    def __repr__(self):
        return f"<Share('{self.ID}', '{self.title}', '{self.date}')>"

The code where I try to remove the entry:

@app.route("/community/share/<share_uuid>/delete")
@login_required
def delete_share(share_uuid):
    share = Share.query.filter_by(uuid=share_uuid).first()
    if share and share.author == current_user:
        pass
    else:
        abort(403)

    if request.args.get("confirm") == "yes":
        db.session.delete(share)
        db.session.commit()

        flash("Share successfully deleted.", "flash")
        return redirect(url_for('community'))

    return render_template("community/share/delete.html", share=share, title="Delete Share?")

What I have tried:

I have no Idea where to start so please send me some help...