implement wagon maintenance feature: add functionality to detach wagons and move them to maintenance status

apps/edr-freight-api/src/modules/trains/train-builder.service.ts

@@ -59,11 +59,7 @@ export class TrainBuilderService {
     }
 
     const trainId = await this.dataSource.transaction(async (manager) => {
-      const code = dto.code.trim();
-      const existing = await manager.getRepository(Train).findOne({ where: { code } });
-      if (existing) {
-        throw new ConflictException(`Train code ${code} is already in use`);
-      }
+      const code = await this.generateTrainCode(manager);
 
       // Friendly 409 before the partial unique indexes (the race-proof backstop):
       // the typed pair may not collide with any train's pair or legacy number.
@@ -418,6 +414,33 @@ export class TrainBuilderService {
     return this.getComposition(id);
   }
 
+  /**
+   * Detach one wagon AND flag it for maintenance: it leaves the consist and
+   * moves to MAINTENANCE status (not AVAILABLE), so it is not re-coupled until
+   * it clears maintenance. The freed sequence gap is closed.
+   */
+  async sendWagonToMaintenance(id: string, wagonId: string) {
+    await this.dataSource.transaction(async (manager) => {
+      const train = await this.getEditableTrain(manager, id);
+      const wagon = await manager.getRepository(Wagon).findOne({ where: { id: wagonId } });
+      if (!wagon || wagon.trainId !== train.id) {
+        throw new NotFoundException(`Wagon ${wagonId} is not part of this train`);
+      }
+      if (wagon.currentTrainScheduleId) {
+        throw new ConflictException(
+          `Wagon ${wagon.wagonNumber} is pinned to an active schedule and cannot be removed`,
+        );
+      }
+      await manager.getRepository(Wagon).update(wagon.id, {
+        trainId: null,
+        sequenceNumber: null,
+        status: WagonStatus.Maintenance,
+      });
+      await this.resequenceWagons(manager, train.id);
+    });
+    return this.getComposition(id);
+  }
+
   /** Persist a drag-reorder: `wagonIds` is the full consist in its new order. */
   async reorderWagons(id: string, dto: ReorderTrainWagonsDto) {
     await this.dataSource.transaction(async (manager) => {
@@ -469,6 +492,29 @@ export class TrainBuilderService {
 
   // ---------------------------------------------------------------- internals
 
+  /**
+   * System-assigned train code `TR-NNNNN`. Draws the next number from the
+   * highest existing `TR-` code and probes past any manual collision so the
+   * unique constraint never rejects the build.
+   */
+  private async generateTrainCode(manager: EntityManager): Promise<string> {
+    const [row]: { max_seq: string | null }[] = await manager.query(
+      `SELECT MAX(CAST(SUBSTRING(code FROM '^TR-([0-9]+)$') AS INTEGER)) AS max_seq
+         FROM freight.trains
+        WHERE code ~ '^TR-[0-9]+$'`,
+    );
+    let seq = Number(row?.max_seq ?? 0) + 1;
+    for (let attempt = 0; attempt < 50; attempt += 1) {
+      const code = `TR-${String(seq).padStart(5, '0')}`;
+      const exists = await manager
+        .getRepository(Train)
+        .findOne({ where: { code }, withDeleted: true });
+      if (!exists) return code;
+      seq += 1;
+    }
+    throw new ConflictException('Could not allocate a unique train code');
+  }
+
   private mapSummary(train: Train, activeSchedule: ActiveScheduleRef | null) {
     const locomotives = [...(train.locomotives ?? [])]
       .sort((a, b) => a.sequenceNo - b.sequenceNo)