Evolution of Defaults

Feb 27, 2025

Stephen Haberman

Joist’s mission is to model your application’s business logic, with first-class support for domain modeling features & concepts.

A great example of this is Joist’s support for something as simple as default values: for example, the Author.status field should default to Active.

Joist’s default values support grew from “the simplest thing possible” (requiring adhoc patterns that engineers would copy/paste around) to a robust, first-class feature (an explicit setDefault API that “just works”).

This is a microcosm of Joist’s goal to identify the repeated patterns and pain points involved in “building a domain model”, and provide elegant features with a great DX.

Version 1. Schema Defaults

Joist’s initial defaults support was purposefully “as simple as possible”, and limited to DEFAULTs declared in the database schema, i.e. an is_archived field that defaults to FALSE, or a status_id that defaults to DRAFT:

CREATE TABLE example_table (
    id SERIAL PRIMARY KEY,
    is_archived BOOL DEFAULT false,
    status_id INTEGER DEFAULT 1,
);

Joist’s codegen would recognize these, and “apply them immediately” when creating an entity:

const a = em.create(Author, {});
expect(a.status).toBe(AuthorStatus.Draft); // already Draft
expect(a.isArchived).toBe(false); // already false

This was super-simple, and had a few pros:

• Pro: The status is immediately within the em.create
  • I.e. you don’t have to wait for an em.flush to “see the database default”
  • Any business logic can immediately start using the default
• Pro: No duplication of “draft is the default” between the database schema & TypeScript code
• Con: Only supports static, hard-coded values
  • Ideally we’d like to write lambdas to calculate defaults, based on business logic

Version 2. beforeCreate hooks

Being limited to static DEFAULT values is not great, so the first way of implementing more complicated “dynamic defaults” was using Joist’s beforeCreate hooks:

/** Any author created w/non-zero amount of books defaults to Published. */
authorConfig.beforeCreate("books", a => {
  if (a.status === undefined) {
    a.status = a.books.get.length > 0 ? AuthorStatus.Published : AuthorStatus.Draft;
  }
})

This was a quick-win b/c Joist already supported beforeCreate hooks, but had a few cons:

• Pro: Supports arbitrary business logic
  • The load hint easily enables cross-entity calculations
• Con: The default logic isn’t ran until em.flush
  • Harder for business logic to rely on
  • Creates inconsistency between “hard-coded defaults” (applied immediately in em.create) and “dynamic defaults” (applied during flush)
• Con: Susceptible to hook ordering issues
  • If our default’s value depends on other defaults, it is hard to ensure the other “runs first”
• Con: Boilerplate/imperative (not really a first-class feature)
  • The code has to 1st check if a.status is already set (not a huge deal, but boilerplate)
  • There is nothing in the code/API that identifies “this is a default”, instead we just have an adhoc pattern of “this is how our app sets defaults”
• Con: Caused duplication with test factories
  • Our test factories often wanted “the same defaults” applied, but Joist’s factories are synchronous, which meant any logic that was “set in beforeCreate” wouldn’t be seen right away.
  • To work around this, we often “wrote twice” default logic across our entities & test factories—not great!

Version 3: Adding setDefault

We lived with the Version 1 & 2 options for several years, because they were “good enough”, but for the 3rd version, we wanted to start “setting defaults” on the road to being “more than just good enough”.

Specifically, we wanted a first-class, idiomatic way to “declaratively specify a field’s default value” instead of the previous “manually check the field in a beforeCreate hook”.

So we added config.setDefault, which accepts the field name, it’s dependencies (if any), and a lambda that would calculate the default value:

/** Calculate the Author.status default, based on number of books. */
authorConfig.setDefault("status", "books", (a) => {
  // a.books.get is available, but a.firstName is not, b/c it's not listed as a dependency
  return a.books.get.length > 0 ? AuthorStatus.Published : AuthorStatus.Draft;
})

This was a great start, but we pushed it out knowingly half-baked:

• Pro: Provided scaffolding of a better future
  • Gave an idiomatic way to “declare defaults”
• Pro: The type system enforces that the lambda only calls fields explicitly listed in the dependency param
  • This reused our ReactiveField infra and is great for ensuring dependencies aren’t missed
• Con: The dependencies weren’t actually used yet
  • “…ship early!”
• Con: setDefault lambdas were still not invoked until em.flush
  • So we still had the “write defaults twice” problem with test factories

Version 4: Dependency Aware

After having the setDefault API in production for a few months, the next improvement was to capitalize on “knowing our dependencies” and allow defaults to depend on other defaults.

For example, maybe our Author.status default needs to know whether any of the books are published (which itself is a default):

// In `Author.ts`
authorConfig.setDefault("status", { books: "status" }, a => {
  const anyBookPublished = a.books.get.some(b => b.status === BookStatus.Published);
  return anyBookPublished ? AuthorStatus.Published : AuthorStatus.Draft;
})

// In `Book.ts`
bookConfig.setDefault("status", {}, b => {
  // Some business logic that dynamically determines the status
  return BookStatus.Published;
});

Now, if both a Book and an Author are created at the same time, em.flush will ensure that the Book.status is calculated before invoking the Author.status default—we’ve solved our ordering issue!

This was a major accomplishment—cross-entity defaults had been a thorn in our side for years.

(Fwiw we readily admit this is a rare/obscure need—in our domain model of 100s of entities, we have only ~2-3 of these “cross-entity defaults”, so we want to be clear this is not necessarily a “must have” feature—but, when you need it, it’s extremely nice to have!)

• Pro: Finally unlocked cross-entity defaults
• Con: Still have the “write defaults twice” problem with factories

Version 5: Teaching Factories!

The next DX iteration was solving the duplication of “factories want the defaults too!”.

Looking more closely at this issue, Joist’s test factories are synchronous, which means we can create test data easily without any awaits:

// Given an author
const a = newAuthor(em);
// And a book
const b = newBook(em, { author: a });
// And setup something else using b.title
// ...if there is "default title logic", it will not have ran yet, which
// can be confusing for tests/other logic expecting that behavior
console.log(b.title);

The lack of awaits is very nice! But it does mean, if we really wanted b.title to immediately reflect its production default, we had recode the default logic into the newBook factory:

export function newBook(em: EntityManager): DeepNew<Book> {
  return newTestInstance(em, Book, {
    title: "recode the Book default logic here",
  });
}

As before, for a while this was “good enough”—but finally in this iteration, we taught the factories to leverage their “each test’s data is already in memory” advantage and just invoke the defaults immediately during the newTestInstance calls.

This works even for setDefaults that use load hints, like “author status depends on its books”:

// In `Author.ts`
authorConfig.setDefault("status", { books: "status" }, a => {
  const anyBookPublished = a.books.get.some(b => b.status === BookStatus.Published);
  return anyBookPublished ? AuthorStatus.Published : AuthorStatus.Draft;
})

In production, Joist can’t assume “the author’s books are already in-memory”, so em.flush would first load / await for the a.books to be loaded, and then invoke the lambda.

However, because our tests know that a.books is already in memory, they can skip this await, and immediately invoke the lambda.

• Pro: We finally can remove the factory’s “write it twice” defaults

Version Next: findOrCreates

Always looking ahead, the next itch we have is that, currently, default lambdas that call async methods like em.find or em.findOrCreate are still skipped during newTestInstance and only run during em.flush.

Which means, for these defaults, we still have remnants of the “write it twice” defaults anti-pattern—albeit very few of them!

We should be able to lift this restriction as well, with a little bit of work (…maybe :thinking:, the newBook call is fundamentally synchronous, so maybe not).

Slow Grind to Perfection

Wrapping up, besides a “walk down memory lane”, the larger point of this post is highlighting Joist’s journey of continually grinding on DX polish—we’re about five years into Joel’s Good Software Takes 10 Years, so only another 5 to go! :smile:

Of course, it’d be great for this evolution to happen more quickly—i.e. if we had a dependency-aware, factory-aware, amazing setDefault API from day one.

But, often times jumping to an abstraction can be premature, and result in a rushed design—so sometimes it doesn’t hurt to “sit with the itch” for a little while, evolve it through multiple iterations of “good enough”, until finally a pleasant/robust solution emerges.

And, perhaps most pragmatically, small iterations helps spread the implementation out over enough hack days that it can actually get shipped. :ship:

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