RelatedViewModelField

Define a field that references another ViewModel

This requires two things:

1) The ViewModel to reference 2) The field on the source ViewModel that contains the ID for the relation

In the following example User has a Group as a relation. The id for the connected group is stored on the groupId field:

class Group extends viewModelFactory({
  name: new CharField(),
}) {}
class User extends viewModelFactory({
  name: new CharField(),
  groupId: new IntegerField(),
  group: new RelatedViewModelField({
    to: Group,
    sourceFieldName: 'groupId',
  }),
}) {}

You can then fetch the data - including the relations - from the cache:

Group.cache.add({ id: 1, name: 'Staff' });
User.cache.add({ id: 1, name: 'Bob', groupId: 1 });
User.cache.get(['name', 'group']);
// { id: 1, name: 'Bob', groupId: 1, group: { id: 1, name: 'Staff' }}

The to field can also be a function to support circular references:

class Group extends viewModelFactory({
  name: new CharField(),
  ownerId: new IntegerField(),
  owner: new RelatedViewModelField({
    to: () => User,
    sourceFieldName: 'ownerId',
  }),
}) {}
class User extends viewModelFactory({
  name: new CharField(),
  groupId: new IntegerField(),
  group: new RelatedViewModelField({
    to: Group,
    sourceFieldName: 'groupId',
  }),
}) {}

You can query the circular relations as deep as you want:

Group.cache.add({ id: 1, name: 'Staff', ownerId: 1 });
User.cache.add({ id: 1, name: 'Bob', groupId: 1 });
User.cache.get(['name', 'group', ['group', 'owner'], ['group', 'owner', 'group']);
// {
//   id: 1,
//   name: 'Bob',
//   groupId: 1,
//   group: {
//     id: 1,
//     name: 'Staff',
//     ownerId: 1,
//     owner: {
//       id: 1,
//       name: 'Bob',
//       groupId: 1,
//       group: {
//         id: 1,
//         name: 'Staff',
//         ownerId: 1,
//       }
//     },
//   },
// }

to can also be a a function that returns a Promise. This is useful to lazy load modules:

class Subscription extends viewModelFactory({
  userId: new IntegerField(),
  user: new RelatedViewModelField({
      sourceFieldName: 'userId',
      to: async () => {
        const User = await import('./User').default;
        return User;
      }
  })
}) {}

NOTE: When you return a promise you have to call resolveViewModel on that field before it's usable:

await Subscription.fields.user.resolveViewModel()

Failure to do this will result in an error being thrown the first time it's accessed.

If you have multiple values use ManyRelatedViewModelField instead.