Issue Description

When loading an entity via SelectQueryBuilder without joining a nullable ManyToOne relation, the returned entity will have the not loaded relation set as null.
Now saving again this entity results in removing any possibly attached related entity.

Expected Behavior

Loading an entity without loading a certain relation should always result in undefined (and not null) for this relation, as undefined can be interpreted as "was not loaded" while null should be "was loaded but is NULL in the database".
Thus saving the entity directly after loading it should be a NO-OP.

Actual Behavior

The not loaded relation is automatically assigned the value null on entity retrieval. Thus saving the entity will remove the link between the entity and the relation. Just loading and saving an entity without actively manipulating it changed the state of the database.

Steps to Reproduce

game.entity.ts with nullable ManyToOne relation creator

import { Entity, JoinColumn, ManyToOne, PrimaryGeneratedColumn } from 'typeorm';
import { User } from './user.entity';

@Entity()
export class Game {

	@PrimaryGeneratedColumn()
	id!: number;

	// some more relations and columns

	@ManyToOne(() => User, user => user.createdGames, { nullable: true })
	@JoinColumn()
	creator!: User | null;

	@Column()
	finished: boolean;

}

user.entity.ts

import { Entity, OneToMany, PrimaryGeneratedColumn } from 'typeorm';
import { Game } from './game.entity';

@Entity()
export class User {

	@PrimaryGeneratedColumn()
	id!: number;

	@OneToMany(() => Game, game => game.creator)
	createdGames!: Game[];

}

game.repository.ts

import { EntityRepository, Repository } from 'typeorm';
import { Game } from './game.entity';

@EntityRepository(Game)
export class GameRepository extends Repository<Game> {}

Retrieval of the entity (just in case it might matter, I added the WHEREs and as comment some joins):

const game: Game | undefined = await this.gameRepository
	.createQueryBuilder('g')
	// several innerJoinAndSelect / leftJoinAndSelect here, but none for g.creator
	.where('g.id = :gameId', { gameId })
	.andWhere('g.finished = FALSE')
	.getOne();

If the game with the given id gameId has a creator set in the database, game.creator will be null. (I guess it will always be null).

Thus running

await this.gameRepository.save(game);

will result in an update of the game in the database, setting the foreign key to its creator to NULL.

My Environment

npm ls typescript also shows 4.3.5 as dependency of @nestjs/cli@8.1.4

Relevant Database Driver(s)

Are you willing to resolve this issue by submitting a Pull Request?

• ✖️ Yes, I have the time, and I know how to start.
• ✖️ Yes, I have the time, but I don't know how to start. I would need guidance.
• ✖️ No, I don’t have the time, but I can support (using donations) development.
• ✅ No, I don’t have the time and I’m okay to wait for the community / maintainers to resolve this issue.