fix(db): use atomic INSERT ON CONFLICT instead of SELECT-then-INSERT

Eliminates race condition under READ COMMITTED isolation and fixes COALESCE sentinel mismatch between the unique index and the application-level query.

Signed-off-by: Prem bharne <prembharne455@gmail.com>

Author: prembharne

backend/src/database/repositories/memberSegmentAffiliationRepository.ts

@@ -44,36 +44,18 @@ class MemberSegmentAffiliationRepository extends RepositoryBase<
 
     const transaction = this.transaction
 
-    const existing = await this.options.database.sequelize.query(
-      `SELECT "id" FROM "memberSegmentAffiliations"
-       WHERE "memberId" = :memberId
-       AND "segmentId" = :segmentId
-       AND "organizationId" IS NOT DISTINCT FROM :organizationId
-       AND "dateStart" IS NOT DISTINCT FROM :dateStart
-       AND "dateEnd" IS NOT DISTINCT FROM :dateEnd
-       LIMIT 1`,
-      {
-        replacements: {
-          memberId: data.memberId,
-          segmentId: data.segmentId,
-          organizationId: data.organizationId,
-          dateStart: data.dateStart || null,
-          dateEnd: data.dateEnd || null,
-        },
-        type: QueryTypes.SELECT,
-        transaction,
-      }
-    )
-
-    if (existing.length > 0) {
-      await this.updateAffiliation(data.memberId, data.segmentId, data.organizationId)
-      return this.findById((existing[0] as any).id)
-    }
-
     const affiliationInsertResult = await this.options.database.sequelize.query(
       `INSERT INTO "memberSegmentAffiliations" ("id", "memberId", "segmentId", "organizationId", "dateStart", "dateEnd")
           VALUES
               (:id, :memberId, :segmentId, :organizationId, :dateStart, :dateEnd)
+          ON CONFLICT (
+              "memberId",
+              "segmentId",
+              COALESCE("organizationId", '00000000-0000-0000-0000-000000000000'::uuid),
+              COALESCE("dateStart", '1970-01-01T00:00:00Z'::timestamptz),
+              COALESCE("dateEnd", '1970-01-01T00:00:00Z'::timestamptz)
+          )
+          DO UPDATE SET "organizationId" = EXCLUDED."organizationId"
           RETURNING "id"
         `,
       {