fix: prevent duplicate project applications

backend/alembic/versions/398a2154b3d5_add_unique_constraint_for_applications.py

@@ -0,0 +1,34 @@
+"""add unique constraint for applications
+
+Revision ID: 398a2154b3d5
+Revises: add_unique_constraint_applicant_project
+Create Date: 2026-07-09 21:40:36.723685
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = '398a2154b3d5'
+down_revision: Union[str, Sequence[str], None] = "c14aa06f723a"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.create_unique_constraint(
+        "uq_applicant_project",
+        "applications",
+        ["applicant_id", "project_id"],
+    )
+
+
+def downgrade() -> None:
+    op.drop_constraint(
+        "uq_applicant_project",
+        "applications",
+        type_="unique",
+    )

backend/app/models/application.py

@@ -11,6 +11,7 @@
     ForeignKey,
     String,
     Text,
+    UniqueConstraint,
     func,
 )
 from sqlalchemy.dialects.postgresql import UUID
@@ -33,7 +34,13 @@ class Application(Base):
     """
 
     __tablename__ = "applications"
-
+    __table_args__ = (
+        UniqueConstraint(
+            "applicant_id",
+            "project_id",
+            name="uq_applicant_project",
+        ),
+    )
     # ==========================================================
     # Primary Key
     # ==========================================================

backend/app/services/application_service.py

@@ -2,7 +2,9 @@
 
 import uuid
 
+from fastapi import HTTPException, status
 from sqlalchemy import select
+from sqlalchemy.exc import IntegrityError
 from sqlalchemy.orm import Session
 
 from app.models.application import (
@@ -28,7 +30,18 @@ def create_application(
         flare_id: uuid.UUID,
         application: ApplicationCreate,
     ) -> Application:
+        existing_application = db.scalar(
+            select(Application).where(
+                Application.applicant_id == applicant_id,
+                Application.project_id == project_id,
+            )
+        )
 
+        if existing_application:
+            raise HTTPException(
+                status_code=status.HTTP_409_CONFLICT,
+                detail="You have already applied to this project.",
+            )
         db_application = Application(
             applicant_id=applicant_id,
             project_id=project_id,
@@ -40,10 +53,17 @@ def create_application(
         )
 
         db.add(db_application)
-        db.commit()
-        db.refresh(db_application)
 
-        return db_application
+        try:
+            db.commit()
+        except IntegrityError:
+            db.rollback()
+            raise HTTPException(
+                status_code=status.HTTP_409_CONFLICT,
+                detail="You have already applied to this project.",
+       )
+        db.refresh(db_application)
+        return db_application    
 
     @staticmethod
     def get_application(