graceful handling of database integrity violations in registration

goathacks/registration/__init__.py

@@ -6,6 +6,7 @@ from goathacks.registration.forms import LoginForm, PwResetForm, RegisterForm, R
 from werkzeug.security import check_password_hash, generate_password_hash
 from flask_mail import Message
 import ulid
+from sqlalchemy.exc import IntegrityError
 
 from goathacks import db, mail as app_mail
 from goathacks.models import PwResetRequest, User
@@ -60,8 +61,20 @@ def register():
                     dietary_restrictions=dietary_restrictions,
                     newsletter=newsletter
             )
-            db.session.add(user)
-            db.session.commit()
+
+            #try to add the user to the database, checking for duplicate users
+            try:
+                db.session.add(user)
+                db.session.commit()
+            except IntegrityError as err:
+                db.session.rollback()
+                if "duplicate key value violates unique constraint" in str(err):
+                    flash("User with email " + email + " already exists.")
+                else:
+                    flash("An unknown error occurred.")
+                return render_template("register.html", form=form)
+
+            #user successfully registered, so login
             flask_login.login_user(user)
 
             if waitlisted:
@@ -129,6 +142,7 @@ def reset():
                     user_id=user.id,
                     expires=datetime.now() + timedelta(minutes=30)
                     )
+            
             db.session.add(r)
             db.session.commit()