sqlalchemy.exc.IntegrityError: (psycopg2.errors.UniqueViolation) duplicate key value violates unique constraint
A database constraint (unique, foreign key, not null) rejected an insert/update made through SQLAlchemy.
Exact message you may see
This Session's transaction has been rolled back
Seen on:
Python
Meaning
The parenthesised driver error names the constraint. After an IntegrityError the session must be rolled back before it can be used again.
Common causes
- Duplicate unique value
- Foreign key pointing to a missing row
- NULL in a NOT NULL column
- Session not rolled back after a previous failure
⚡ Quick fix
- Catch IntegrityError, session.rollback(), and report “already exists”
- Validate references before inserting
- Use upserts (insert().on_conflict_do_update())
Detailed fix by platform
Python
- python
try: db.session.commit() except IntegrityError: db.session.rollback() raise
How to diagnose
- Constraint — Which one is named?
- Session — Rolled back after errors?
🔧 Still not fixed?
Many errors look alike. If the steps above didn’t solve it, one of these is probably what you’re facing:
Similar errors
Most viewed in Python
Other ways to find it
Where this error sits
Origin: Application & API. Errors that are often confused with or caused by this one (click to open):
🧠 Still stuck? Analyze your error
Paste the full message, response headers or stack trace — we'll detect the platform and point to the most likely cause.
💬 What worked for others
No tips yet — did something else fix this for you?
Share what worked for you
Tips are checked by an editor before they appear. Links are removed.
Did this fix work for you?
Report a correction or suggest an improvement
Last updated 12 Oct 2026