feat(poll): enforce active respondent uniqueness

src/govoplan_poll/backend/db/models.py

@@ -99,6 +99,18 @@ class PollResponse(Base, TimestampMixin):
     __table_args__ = (
         Index("ix_poll_responses_poll_submitted", "poll_id", "submitted_at"),
         Index("ix_poll_responses_poll_respondent", "poll_id", "respondent_id"),
+        Index(
+            "uq_poll_responses_active_respondent",
+            "poll_id",
+            "respondent_id",
+            unique=True,
+            sqlite_where=text(
+                "deleted_at IS NULL AND respondent_id IS NOT NULL"
+            ),
+            postgresql_where=text(
+                "deleted_at IS NULL AND respondent_id IS NOT NULL"
+            ),
+        ),
         Index("ix_poll_responses_tenant_poll", "tenant_id", "poll_id"),
     )
 

src/govoplan_poll/backend/migrations/versions/6e7f8a9b0c1d_v0110_poll_response_uniqueness.py

@@ -0,0 +1,99 @@
+"""v0.1.10 active identified Poll response uniqueness
+
+Revision ID: 6e7f8a9b0c1d
+Revises: 5d6e7f8a9b0c
+Create Date: 2026-07-22 00:00:00.000000
+
+This migration deduplicates active identified responses before creating the
+partial unique index. Run it while Poll response writes are quiesced or while
+the platform maintenance lock is held. PostgreSQL additionally takes a table
+lock so an unexpected writer cannot enter between cleanup and index creation.
+"""
+from __future__ import annotations
+
+from alembic import op
+import sqlalchemy as sa
+
+
+revision = "6e7f8a9b0c1d"
+down_revision = "5d6e7f8a9b0c"
+branch_labels = None
+depends_on = None
+
+
+_INDEX_NAME = "uq_poll_responses_active_respondent"
+_ACTIVE_IDENTIFIED_PREDICATE = (
+    "deleted_at IS NULL AND respondent_id IS NOT NULL"
+)
+
+
+def _tombstone_duplicate_active_responses() -> None:
+    responses = sa.table(
+        "poll_responses",
+        sa.column("id", sa.String(length=36)),
+        sa.column("poll_id", sa.String(length=36)),
+        sa.column("respondent_id", sa.String(length=255)),
+        sa.column("submitted_at", sa.DateTime(timezone=True)),
+        sa.column("deleted_at", sa.DateTime(timezone=True)),
+        sa.column("updated_at", sa.DateTime(timezone=True)),
+    )
+    ranked = (
+        sa.select(
+            responses.c.id.label("id"),
+            sa.func.row_number()
+            .over(
+                partition_by=(
+                    responses.c.poll_id,
+                    responses.c.respondent_id,
+                ),
+                order_by=(
+                    responses.c.submitted_at.desc(),
+                    responses.c.id.desc(),
+                ),
+            )
+            .label("response_rank"),
+        )
+        .where(
+            responses.c.deleted_at.is_(None),
+            responses.c.respondent_id.is_not(None),
+        )
+        .subquery("ranked_active_poll_responses")
+    )
+    duplicate_ids = sa.select(ranked.c.id).where(ranked.c.response_rank > 1)
+    migration_timestamp = sa.func.current_timestamp()
+    op.get_bind().execute(
+        responses.update()
+        .where(
+            responses.c.id.in_(duplicate_ids),
+            responses.c.deleted_at.is_(None),
+        )
+        .values(
+            deleted_at=migration_timestamp,
+            updated_at=migration_timestamp,
+        )
+    )
+
+
+def upgrade() -> None:
+    connection = op.get_bind()
+    if connection.dialect.name == "postgresql":
+        connection.execute(
+            sa.text(
+                "LOCK TABLE poll_responses IN SHARE ROW EXCLUSIVE MODE"
+            )
+        )
+    _tombstone_duplicate_active_responses()
+    op.create_index(
+        _INDEX_NAME,
+        "poll_responses",
+        ["poll_id", "respondent_id"],
+        unique=True,
+        sqlite_where=sa.text(_ACTIVE_IDENTIFIED_PREDICATE),
+        postgresql_where=sa.text(_ACTIVE_IDENTIFIED_PREDICATE),
+    )
+
+
+def downgrade() -> None:
+    # Deduplicated rows remain tombstoned: a downgrade must not silently revive
+    # responses that were no longer part of the active result set.
+    op.drop_index(_INDEX_NAME, table_name="poll_responses")