Question: I have two tables created with Flask-SQLAlchemy below - they have a one to one relationship. class Logo(db.Model): __tablename__ = "logo" id = db.Column(db.Integer, primary_key=True) filename = db.Column(db.String(100)) data = db.Column(db.LargeBinary) username = db.Column(db.String(100), db.ForeignKey("users.username")) users = db.relationship("User", backref=backref("logo", uselist=False)) def __init__(self, filename: str, data, username: str): self.filename = filename self.data = data self.username = username def __repr__(self) -> str: return "<Logo (filename='{}', username='{}')>".format( self.filename, self.username ) class User(UserMixin, db.Model): __tablename__ = "users" id = db.Column(db.Integer, primary_key=True) username = db.Column(db.String(100), unique=True) password = db.Column( db.String(200), primary_key=False, unique=False, nullable=False ) is_admin = db.Column(db.Boolean, default=False, nullable=True) def __init__( self, username: str, password: str, is_admin: bool = False, ): self.username = username self.password = self.set_password(password) self.is_admin = is_admin def get_id(self): return self.username def set_password(self, password: str) -> str: return generate_password_hash(password, method="sha256") def check_password(self, password: str): return check_password_hash(self.password, password) def __repr__(self) -> str: return "<User {}>".format(self.username)  I would like to update the user table in a case when the user would like to have a new username: user01 = User.query.filter_by(username="user01").first() logo = Logo.query.filter_by(username="user01").first() new_username= "newusertest" user01.username = new_username logo.users = user01 logo.username = new_username db.session.add(user01) db.session.add(logo) db.session.commit()  The db.session.commit throws the following error: IntegrityError: (psycopg2.errors.ForeignKeyViolation) update or delete on table "users" violates foreign key constraint "logo_username_fkey" on table "logo" DETAIL: Key (username)=(user01) is still referenced from table "logo". [SQL: UPDATE users SET username=%(username)s WHERE users.id = %(users_id)s] [parameters: {'username': 'newusertest', 'users_id': 2}] (Background on this error at: https://sqlalche.me/e/14/gkpj)  The error says the logo table still has the old username but I have updated it and I don't know why that shows up again, I have spent the last 2 hours debugging and trying different stuff but nothing works.

Answer: You could temporarily make the foreign key constraint deferrable and make the update in psql. Say we have these tables: test# \d parent Table "public.parent" Column │ Type │ Collation │ Nullable │ Default ════════╪═══════════════════╪═══════════╪══════════╪══════════════════════════════ id │ integer │ │ not null │ generated always as identity name │ character varying │ │ │ Indexes: "parent_name_key" UNIQUE CONSTRAINT, btree (name) Referenced by: TABLE "child" CONSTRAINT "child_pname_fkey" FOREIGN KEY (pname) REFERENCES parent(name) test# \d child Table "public.child" Column │ Type │ Collation │ Nullable │ Default ════════╪═══════════════════╪═══════════╪══════════╪══════════════════════════════ id │ integer │ │ not null │ generated always as identity pname │ character varying │ │ │ Foreign-key constraints: "child_pname_fkey" FOREIGN KEY (pname) REFERENCES parent(name)  then the statements would be test# alter table child alter constraint child_pname_fkey deferrable; ALTER TABLE SET CONSTRAINTS test# begin; BEGIN test#* set constraints child_pname_fkey deferred; SET CONSTRAINTS test#* update child set pname = 'Alice' where id = 1; UPDATE 1 test#* update parent set name = 'Alice' where id = 1; UPDATE 1 test#* commit; COMMIT test# alter table child alter constraint child_pname_fkey not deferrable; ALTER TABLE test#  Deferring the constraint means updates are evaluated at the end of the transaction rather than immediately, so the the point of view of the database the columns are not out of sync. The long term solution is to use users.id as the foreign key, as it is less likely to change.