OptionalProps in Mikro-orm

Question

I am trying to work out how to define additional optional properties.

import { Entity, PrimaryKey, Property, OptionalProps } from '@mikro-orm/core';

@Entity()
export abstract class BaseEntity {
  [OptionalProps]?: 'createdAt';

  @PrimaryKey()
  id: number;

  @Property()
  createdAt: Date = new Date();

}

@Entity()
export class EntityA extends BaseEntity {
  [OptionalProps]?: 'isAnotherProperty'; // This is the bit I cannot figure out

  @Property()
  isAnotherProperty: boolean = false;

}

With the above TypeScript throws an error:

Property '[OptionalProps]' in type 'EntityA' is not assignable to the same property in base type 'BaseEntity'.

Basically my BaseEntity has optional properties, as does EntityA. I could remove [OptionalProps]?: from BaseEntity and have [OptionalProps]?: 'createdAt' | 'isAnotherProperty'; in EntityA, but many of my entities don't require any additional optional properties beyond createdAt so I prefer not to have to duplicate [OptionalProps]?: 'createdAt'; in every entity class if I could just 'extend' it where I need to.

Is it at all possible to either append to or override [OptionalProps]?

Answer 1

2023 update

The upcoming v6 will have another way to mark optional properties on type-level - the Opt type, which does not suffer from the extension issues. You can use in two ways (and it won't :

import { Entity, PrimaryKey, Property, Opt } from '@mikro-orm/core';

@Entity()
export abstract class BaseEntity {

  @PrimaryKey()
  id: number;

  @Property()
  createdAt: Date & Opt = new Date(); // either intersect

}

@Entity()
export class EntityA extends BaseEntity<'isAnotherProperty'> {

  @Property()
  isAnotherProperty: Opt<boolean> = false; // or use it as a generic

}

This approach can be combined with the OptionalProps symbol.

https://mikro-orm.io/docs/next/guide/relationships#alternative-approach-with-opt-type

---

Older (but still valid) approach

Probably the cleanest approach is via type argument on the base entity:

import { Entity, PrimaryKey, Property, OptionalProps } from '@mikro-orm/core';

@Entity()
export abstract class BaseEntity<Optional = never> {

  [OptionalProps]?: Optional | 'createdAt';

  @PrimaryKey()
  id: number;

  @Property()
  createdAt: Date = new Date();

}

@Entity()
export class EntityA extends BaseEntity<'isAnotherProperty'> {

  @Property()
  isAnotherProperty: boolean = false;

}