fix issue

apps/edr-freight-api/src/modules/locomotives/locomotives.service.ts

@@ -1,4 +1,5 @@
 import { ConflictException, Injectable, NotFoundException } from '@nestjs/common';
+import { DataSource } from 'typeorm';
 
 import { CreateLocomotiveDto } from './dto/create-locomotive.dto';
 import { FilterLocomotivesDto } from './dto/filter-locomotives.dto';
@@ -9,11 +10,23 @@ import {
   type LocomotiveStatus,
   type LocomotiveType,
 } from './entities/locomotive.entity';
+import { TrainLocomotive } from '../trains/entities/train-locomotive.entity';
 import { LocomotivesRepository } from './locomotives.repository';
 
 @Injectable()
 export class LocomotivesService {
-  constructor(private readonly locomotivesRepository: LocomotivesRepository) {}
+  constructor(
+    private readonly locomotivesRepository: LocomotivesRepository,
+    private readonly dataSource: DataSource,
+  ) {}
+
+  /** The built-train link (if any) coupling this locomotive to a fleet train. */
+  private findTrainLink(locomotiveId: string): Promise<TrainLocomotive | null> {
+    return this.dataSource.getRepository(TrainLocomotive).findOne({
+      where: { locomotiveId },
+      relations: { train: true },
+    });
+  }
 
   findAll(filter: FilterLocomotivesDto): Promise<Locomotive[]> {
     return this.locomotivesRepository.findAll({
@@ -94,6 +107,22 @@ export class LocomotivesService {
       }
     }
 
+    // A locomotive coupled to a built train follows the train: its yard and
+    // status are owned by the train-builder flow, not this generic PATCH.
+    const link = await this.findTrainLink(id);
+    if (link) {
+      if (dto.currentYardId !== undefined && dto.currentYardId !== link.train?.currentYardId) {
+        throw new ConflictException(
+          `Locomotive ${locomotive.code} is coupled to train ${link.train?.code}; move the train (train-builder yard change) instead`,
+        );
+      }
+      if (dto.status !== undefined && dto.status !== locomotive.status) {
+        throw new ConflictException(
+          `Locomotive ${locomotive.code} is coupled to train ${link.train?.code}; detach it before changing its status`,
+        );
+      }
+    }
+
     const updated = await this.locomotivesRepository.update(id, {
       ...dto,
       locomotiveType:
@@ -119,7 +148,16 @@ export class LocomotivesService {
   }
 
   async decommission(id: string): Promise<Locomotive> {
-    await this.findById(id);
+    const locomotive = await this.findById(id);
+
+    // Can't retire a locomotive that is still coupled to a built train — detach
+    // it in the train-builder first so the train never loses a live loco.
+    const link = await this.findTrainLink(id);
+    if (link) {
+      throw new ConflictException(
+        `Locomotive ${locomotive.code} is coupled to train ${link.train?.code}; detach it before taking it out of service`,
+      );
+    }
 
     const updated = await this.locomotivesRepository.update(id, {
       status: 'OUT_OF_SERVICE',