Nestjs: How to use mongoose to start a session for transaction?

Question

The mongoose documentation to use a transaction is straightforward but when it is followed in nestjs, it returns an error:

Connection 0 was disconnected when calling `startSession`
MongooseError: Connection 0 was disconnected when calling `startSession`
    at NativeConnection.startSession

My code:

const transactionSession = await mongoose.startSession();
    transactionSession.startTransaction();

    try
    {
      const newSignupBody: CreateUserDto = {password: hashedPassword, email, username};
  
      const user: User = await this.userService.create(newSignupBody);

      //save the profile.
      const profile: Profile = await this.profileService.create(user['Id'], signupDto);

      const result:AuthResponseDto = this.getAuthUserResponse(user, profile);

      transactionSession.commitTransaction();
      return result;
    }
    catch(err)
    {
      transactionSession.abortTransaction();
    }
    finally
    {
      transactionSession.endSession();
    }

Answer 1

I found the solution after studying @nestjs/mongoose. The mongoose here has no connection in it. This is the reason of error being returned.

The solution:

import {InjectConnection} from '@nestjs/mongoose';
import * as mongoose from 'mongoose';

In the constructor of the service class, we need add connection parameter that can be used by the service.

export class AuthService {
constructor(
  // other dependencies...
  @InjectConnection() private readonly connection: mongoose.Connection){}

Instead of

const transactionSession = await mongoose.startSession();
transactionSession.startTransaction();

We will now use:

const transactionSession = await this.connection.startSession();
transactionSession.startTransaction();

This way, the issue of disconnection after startSession() can be resolved.