Deletion

Facts are immutable, so you cannot really delete them. Instead, you can create a new fact that marks its predecessor as deleted.

class ProjectDeleted {
  static Type = "Construction.Project.Deleted" as const;
  type = ProjectDeleted.Type;

  constructor(public project: Project, public deletedAt: Date) {}
}

const projectADeleted = await j.fact(new ProjectDeleted(projectA, new Date()));

Don't forget to include the ProjectDeleted type in your model:

const constructionModel = (b: ModelBuilder) => b
  .type(Project, m => m
    .predecessor("creator", User)
  )
  .type(ProjectDeleted, m => m
    .predecessor("project", Project)
  )
  ;

On its own, this fact does not do anything.

const projects = await j.query(projectsCreatedByUser, user);

But if you include it in the specification, it will filter out the predecessor from the results.

const projectsCreatedByUser = model.given(User).match(u =>
  u.successors(Project, p => p.creator)
    .notExists(p => p.successors(ProjectDeleted, d => d.project))
);

const projects = await j.query(projectsCreatedByUser, user);

Now if you are really concerned about facts never being truly deleted, there is a way to purge unreachable facts. We'll save that for later.

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