[Question] Transaction scope

[jays1204]

Hi I have a question about transaction scope.

below is some part of my nestjs application code.

  constructor(
    private readonly entityManager: EntityManager
  ) {}

  async saveMyPost(text) {
    const post = new Post(); // Post is Entity
    post.content = text;
    await this.entityManager.persistAndFlush(post);
  }

  async myMethod() {
    await this.saveMyPost('THIS_WILL_BE_SAVE');
    await this.entityManager.transactional(async em => {
      const user = new User();
      user.name = 'myName';
      em.persist(user);

      await this.saveMyPost('THIS_WILL_NOT_BE_SAVE');

      if (true) {
        throw new Error('CUSTOM_ERROR');
      }
      await em.flush();
    });
  }

If I call myMethod(), then I expected 2 rows created on Post. (THIS_WILL_BE_SAVE, THIS_WILL_NOT_BE_SAVE)
because I think saveMyPost method have different database session(connection).
But just 1 row created on Post table. (THIS_WILL_BE_SAVE)

every database query in entityManager.transactional callback method is wrapped single transaction? likes @Transaction decorator of typeorm or @Transactinoal annotation of Spring.

How is this possible? Each http request keeps each database connection until response?
I want to understand about how it works.
So Is there a document that I can understand about this part? Or I want to know the location of the code about that.