feat(core): add Rel<T> and Ref<T> relation types

- `Rel<T>` is an identity type that can be used to get around issues
  with cycles in bidirectional relations, mainly when using ESM together
  with reflect-metadata.
- `Ref<T, PK>` is a shortcut for `IdentifiedReference`.

packages/core/src/index.ts

@@ -7,7 +7,7 @@ export {
   Constructor, ConnectionType, Dictionary, PrimaryKeyType, PrimaryKeyProp, Primary, IPrimaryKey, ObjectQuery, FilterQuery, IWrappedEntity, EntityName, EntityData, Highlighter,
   AnyEntity, EntityClass, EntityProperty, EntityMetadata, QBFilterQuery, PopulateOptions, Populate, Loaded, New, LoadedReference, LoadedCollection, IMigrator, IMigrationGenerator,
   GetRepository, EntityRepositoryType, MigrationObject, DeepPartial, PrimaryProperty, Cast, IsUnknown, EntityDictionary, EntityDTO, MigrationDiff,
-  IEntityGenerator, ISeedManager, EntityClassGroup, OptionalProps, RequiredEntityData, CheckCallback, SimpleColumnMeta,
+  IEntityGenerator, ISeedManager, EntityClassGroup, OptionalProps, RequiredEntityData, CheckCallback, SimpleColumnMeta, Rel, Ref,
 } from './typings';
 export * from './enums';
 export * from './errors';

packages/core/src/typings.ts

@@ -199,6 +199,13 @@ export type EntityDictionary<T> = EntityData<T> & Dictionary;
 type Relation<T> = {
   [P in keyof T as T[P] extends unknown[] | Record<string | number | symbol, unknown> ? P : never]?: T[P]
 };
+
+/** Identity type that can be used to get around issues with cycles in bidirectional relations. */
+export type Rel<T> = T;
+
+/** Shortcut for `IdentifiedReference`. */
+export type Ref<T, PK extends keyof T | unknown = PrimaryProperty<T>> = IdentifiedReference<T, PK>;
+
 export type EntityDTOProp<T> = T extends Scalar
   ? T
   : T extends LoadedReference<infer U>