Mutable Properties

Facts are immutable. So you don't want to put a property like name in a project. You would never be able to change it!

Instead, you can define a successor that represents changing the project name. This is the pattern we use.

class ProjectName {
  static Type = "Construction.Project.Name" as const;
  type = ProjectName.Type;

  constructor(
    public project: Project,
    public value: string,
    public prior: ProjectName[]
  ) { }
}

That prior array let's you overwrite a prior value. The first value will have an empty array, which means there is no prior value.

const projectAName1 = await j.fact(new ProjectName(projectA, "Cheyenne Expansion", []));

To change the name, you create a new fact that has the old name as its prior value.

const projectAName2 = await j.fact(new ProjectName(projectA, "Rivercrest Expansion", [projectAName1]));

To query for the current name, you look for a successor that does not appear in any list of prior values.

const namesOfProject = model.given(Project).match(p =>
  p.successors(ProjectName, n => n.project)
    .notExists(n => n.successors(ProjectName, next => next.prior))
);

const names = await j.query(namesOfProject, projectA);

names.map(n => n.value)

[ "Rivercrest Expansion" ]

Concurrent Edits

The reason that we use the prior array is to handle concurrent edits. If a user on a different device changes the name of the project, you will get a fork in the graph.

const projectAName3 = await j.fact(new ProjectName(projectA, "Cheyenne Remodel", [projectAName1]));

That will result in more than one successor matching the specification.

const names = await j.query(namesOfProject, projectA);

names.map(n => n.value)

[ "Rivercrest Expansion", "Cheyenne Remodel" ]

This is how you can recognize that there have been concurrent edits. It also shows you the candidate values for the project name.

To merge, identify the correct value. Then create a new fact that sets the correct value and has all candidates in its prior array.

const projectAName4 = await j.fact(new ProjectName(projectA, "Rivercrest Remodel", names));

And now only one successor matches the specification.

const names = await j.query(namesOfProject, projectA);

names.map(n => n.value)

[ "Rivercrest Remodel" ]