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backend/alembic/versions/a2b3c4d5e6f7_add_unique_constraint_friendship.py

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+"""add_unique_constraint_friendship
+
+Revision ID: a2b3c4d5e6f7
+Revises: f4e8a1b2c3d9
+Create Date: 2026-03-29 00:00:00.000000
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+from sqlalchemy import text
+
+
+# revision identifiers, used by Alembic.
+revision: str = 'a2b3c4d5e6f7'
+down_revision: Union[str, Sequence[str], None] = 'f4e8a1b2c3d9'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    # Remove duplicate (requester_id, addressee_id) pairs that already exist,
+    # keeping the earliest row per pair before adding the unique constraint.
+    conn = op.get_bind()
+    conn.execute(text("""
+        DELETE FROM friendships
+        WHERE id IN (
+            SELECT id FROM (
+                SELECT id,
+                       ROW_NUMBER() OVER (
+                           PARTITION BY requester_id, addressee_id
+                           ORDER BY created_at
+                       ) AS rn
+                FROM friendships
+            ) sub
+            WHERE rn > 1
+        )
+    """))
+
+    op.create_unique_constraint(
+        "uq_friendship_requester_addressee",
+        "friendships",
+        ["requester_id", "addressee_id"],
+    )
+
+
+def downgrade() -> None:
+    op.drop_constraint("uq_friendship_requester_addressee", "friendships", type_="unique")