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Derived Fields

Derived fields are field values (i.e. primitives) that can be calculated from other data within your domain model.

There are two axis by which to categorize derived fields:

  • Synchronous vs. Asynchronous

    Synchronous derived fields only rely on other fields/columns within the same entity (i.e. an Authors initials only require knowing the same Author's firstName and lastName), so do not need to load any other entities to calculate their value.

    Async derived values do rely on fields from other entities (i.e. an Author's number of books requires loading the author.books one-to-many collection), and so need to load references/collections before calculating their value.

  • Persisted vs. Unpersisted

    Persisted derived fields have their latest value stored in the database, typically for quick access on summary screens, filtering, or exposing the values to external systems like a data warehouse.

    Unpersisted derived fields are not stored in the database, and have their value recalculated every time they're accessed.

And so there are 4 combinations (async vs. sync x persisted vs. unpersisted), and Joist has an approach for modeling each combination.

Synchronous, Unpersisted Fields

For synchronous, unpersisted fields, you can just implement these as getters in your domain objects:

export class Author {
/** Implements the business logic for an unpersisted derived value. */
get fullName(): string {
return this.firstName + (this.lastName ? ` ${this.lastName}` : "");

Synchronous, Persisted Fields

For synchronous, persisted fields, there will be a column in the database to hold the value, i.e. authors.initials, which you can mark as sync in joist-config.json:

"entities": {
"Author": {
"fields": {
"initials": { "derived": "sync" }

This will cause the Author.initials field to not have a setter, only an abstract getter than you must implement:

export class Author {
/** Implements the business logic for a (synchronous) persisted derived value. */
get initials(): string {
return (this.firstName || "")[0] + (this.lastName !== undefined ? this.lastName[0] : "");

This getter will be automatically called by Joist during any INSERT or UPDATE of Author, to determine the latest value.

Because of this, synchronous persisted derived fields should be cheap to calculate.

Asynchronous, Unpersisted Fields

For async, unpersisted fields, you can use hasAsyncProperty:

export class Author {
/** Example of an async property that can be loaded via a populate hint. */
readonly numberOfBooks: AsyncProperty<Author, number> = hasAsyncProperty("books", (a) => {
return a.books.get.length;

Because it's async, the property must be loaded with a populate hint:

const a = await em.load(Author, "a:1", "numberOfBooks");

Asynchronous, Persisted Fields

For async, persisted fields, there will be a column in the database to hold the value, i.e. authors.number_of_books, which you can mark as async in joist-config.json:

"entities": {
"Author": {
"fields": {
"numberOfBooks": { "derived": "async" }

And then implement a property in the Author domain model with the same name:

import { ReactiveField, hasReactiveField } from "joist-orm";

class Author extends AuthorCodegen {
readonly numberOfBooks: ReactiveField<Author, number> = hasReactiveField(
(author) => author.books.get.length,

The readonly property must be of type ReactiveField, which has two type arguments:

  1. The type of the entity
  2. The type of the property

The value is the result of the method hasReactiveField that has three arguments:

  • fieldName: The name of the property, this should match the name of the field in the entity and in joist-config.json.
  • reactiveHint: The name of the fields on the entity that should trigger a recalculation of the derived field. This can be a string("books"), an array of strings (["books", "someOtherRelationship"]) or an object of nested relationships ({books: ["reviews"]}).
  • fn The function that calculates the value of the derived field. This function will be called with the entity as the only argument. All of the fields in the reactiveHint will be loaded before this function is called and can be accessed syncronously using get.

Joist will call this lambda:

  1. When the Author is initially created
  2. When the Author is updated
  3. Whenever one of the Author's books changes
  4. When a Book is created or deleted for the Author

For example, in this scenario:

const a1 = await em.load(Author, "a:1");
const a2 = await em.load(Author, "a:2");

const b1 = em.create(Book, { author: a1 });

em.flush(); // automatically updates a1.numberOfBooks