@Entity()
class User {
  @Property({nullable: true, type: JsonType})
  children?: Child[];

  @Property({ type: ArrayType, nullable: true, persist: true, hydrate: false })
  get childEmails(): string[] | undefined {
    return this.children?.map((c) => c.email.toLowerCase()).filter((e) => !!e);
  }
}

import { ArrayType, Entity, JsonType, PrimaryKey, Property } from '@mikro-orm/postgresql';

export type Child = {
  email: string;
}
@Entity()
export class User {

  @PrimaryKey()
  id!: number;

  @Property({nullable: true, type: JsonType})
  children?: Child[];

  @Property({ type: ArrayType, nullable: true, persist: true, hydrate: false })
  get childEmails(): string[] | undefined {
    return this.children?.map((c) => c.email.toLowerCase()).filter((e) => !!e);
  }
}

import { MikroORM } from '@mikro-orm/sqlite';
import { User } from './entity';

let orm: MikroORM;

beforeAll(async () => {
  orm = await MikroORM.init({
    dbName: ':memory:',
    entities: [User],
    debug: ['query', 'query-params'],
    allowGlobalContext: true, // only for testing
  });
  await orm.schema.refreshDatabase();

  orm.em.create(User, {
    children: [{
      email: 'test@example.com'
    }],
  });
  await orm.em.flush();
  orm.em.clear();

  const connection = orm.em.getConnection();
  dbExecuteSpy = jest.spyOn(connection, 'execute');
});

let dbExecuteSpy: jest.SpyInstance;

beforeEach(() => {
  dbExecuteSpy.mockClear();
});

afterAll(async () => {
  await orm.close(true);
});

test('should not try to persist persisted getter if its value has not changed', async () => {

  await orm.em.findAll(User);

  await orm.em.flush();
  
  expect(dbExecuteSpy).toHaveBeenCalledTimes(1);

  expect(dbExecuteSpy).toHaveBeenNthCalledWith<any[]>(
    1,
    'select `u0`.* from `user` as `u0`',
    [],
    'all',
    undefined,
    { id: 1 },
  );
});

import { MikroORM, defineConfig } from '@mikro-orm/postgresql';
import { User } from './entity';

let orm: MikroORM;

beforeAll(async () => {
  orm = await MikroORM.init(defineConfig({
    dbName: 'postgres',
    entities: [User],
    debug: ['query', 'query-params'],
    allowGlobalContext: true, // only for testing
  }));
  await orm.schema.refreshDatabase();

  orm.em.create(User, {
    children: [{
      email: 'test@example.com'
    }],
  });
  await orm.em.flush();
  orm.em.clear();

  const connection = orm.em.getConnection();
  dbExecuteSpy = jest.spyOn(connection, 'execute');
});

let dbExecuteSpy: jest.SpyInstance;

afterAll(async () => {
  await orm.close(true);
});

test('should not try to persist persisted getter if its value has not changed', async () => {
  dbExecuteSpy.mockClear();

  await orm.em.findAll(User);

  await orm.em.flush();
  
  expect(dbExecuteSpy).toHaveBeenCalledTimes(2); // This is unexpected, it should be 1

  expect(dbExecuteSpy).toHaveBeenNthCalledWith<any[]>(
    1,
    'select \"u0\".* from \"user\" as \"u0\"',
    [],
    'all',
    undefined,
    { id: 1 },
  );

  // This is the additional query that is being done
  expect(dbExecuteSpy).toHaveBeenNthCalledWith<any[]>(
    2,
    'update \"user\" set \"child_emails\" = ? where \"id\" = ?',
    [
        "{test@example.com}",
        1
    ],
    'run',
    expect.anything(),
    { id: undefined },
  );
});