Are you an instance or a plain object?

Firestore is one of those tools that feels like cheating (in a good way). You can build something practical fast: collections, documents, transactions, done. It’s the engineering equivalent of instant ramen… until the day the ramen bites back.

The bite usually sounds like this:

Firestore doesn’t support JavaScript Objects with custom prototypes.

The sentence is Firestore’s way of saying:

“I store data. Not your beautiful class instances.” — and fair enough.

The crime scene

When this error first appeared, my first assumption was the usual one:

“Oops, I probably passed a domain entity into Firestore.”

But nope. I was careful about boundaries. The domain was separated from persistence. The object I passed wasn’t a domain entity but a DB model class (a DTO-like class representing the Firestore document).

It looked innocent:

await tx.update(ref, companyDbDto); // companyDbDto is a DB model class instance
// Firestore: custom prototypes are not serializable

This is where the philosophical question appears:

“Are you an instance… or a plain object”?

Firestore only wants the latter.

The deeper mismatch: Domain ↔ DbDto ↔ POJO

The real issue wasn’t “domain leakage”. It was that my persistence layer had an extra representation. I had a rich domain class (Company) with invariants and methods, a DB model class (CompanyDbDto), and then what Firestore actually wants: plain objects (POJOs).

So the real pipeline was:

Company (domain class) ↔ CompanyDbDto (DB class) ↔ Plain Object (POJO for Firestore)

Having Domain ↔ DB is normal. But adding DB class ↔ POJO as a mandatory step is… annoying.

This introduces a new kind of bug: you can forget the extra conversion, or assume “the converter will handle it,” only to have update() reject it anyway.

Why converters didn’t help

Firestore converters are useful. They feel like the missing ORM piece: “give Firestore a model, and it’ll serialize/deserialize for you.”

That intuition is half-true, which is the most dangerous kind of truth.

The important detail is this:

• Some Firestore operations are “store/retrieve an object” flavored
• update() is “apply this patch” flavored

And patch-flavored APIs don’t compose well with class instances. update() expects plain update data: field paths, primitive values, Firestore FieldValue operations, etc. It’s not trying to “store an object” — it’s trying to apply a diff.

At some point I started using this workaround:

await ref.set(partialData, { merge: true });

It works, but the naming bothered me. In my head, update() means “update existing” while set(..., { merge: true }) feels like an upsert that creates if missing and merges otherwise. It’s practical, but semantically slippery if you aren’t explicit.

The uncomfortable options

Three paths forward emerged, each with tradeoffs:

Option A

Keep DB models as classes, and convert manually before writing. Something like:

DbDto class instance → instanceToPlain(...) → Firestore update/set

This “works,” but it’s fragile. You have to remember the conversion step in every place that writes. So the architecture becomes “correct only if everyone is careful forever.” That’s not a policy — it’s wishful thinking.

Option B

Just use set(..., { merge: true }) everywhere. This also works, but now your repository has fuzzy semantics unless you deliberately encode them (create-only, update-only, upsert, etc.). Otherwise you can silently create docs when you meant to update.

Option C

Make persistence models match what Firestore actually stores. This was the cleanest: make the persistence representation a plain object type, not a class, and validate it at runtime.

Which leads to the rule that fixed everything.

The design rule I adopted

I ended up adopting a simple mental model:

1. Firestore stores POJOs. So persistence “models” should be types / plain objects, not classes.
2. Domain can stay rich. Domain entities can be classes, because they don’t go into Firestore directly.
3. Validate Firestore reads at the boundary. Firestore is schemaless, so runtime validation is a feature, not bureaucracy.
4. Make write semantics explicit. create() should fail if exists. update() should enforce invariants. Upsert should be a deliberate choice.

I did consider the alternative pattern where domain entities implement something like toPrimitives() and the converter becomes “rich.” That can work too. But since my domain boundary wasn’t the problem, I preferred keeping the domain layer totally persistence-agnostic.

Two diagrams

Dependencies

Who imports whom:

Service → Repository → (Mapper + DocSchema + Firestore Adapter)
Domain (Company) depends on nothing

Data flow

What transforms into what:

READ:  Firestore POJO → Zod parse → CompanyDoc → map → Company
WRITE: Company → map → CompanyDoc → POJO → Firestore

Separating dependency direction from data flow made the design finally click.

A concrete example

The core idea is: validate Firestore data into a doc schema, and map doc ↔ domain explicitly.

1. Doc schema

Define the Firestore doc schema using Zod. Keep it plain — no classes required.

import { z } from 'zod';

export const CompanyDocSchema = z.object({
  name: z.string().min(1),
  industry: z.enum([
    'technology',
    'finance',
    'healthcare',
    // ...
  ]),
  status: z.enum(['pending_approval', 'active' /* ... */]),
  isPublic: z.boolean(),
  createdAt: z.string(),
  updatedAt: z.string(),

  // optional fields
  description: z.string().optional(),
  website: z.url().optional(),
});

export type CompanyDoc = z.infer<typeof CompanyDocSchema>;

2. Mapper

Convert between CompanyDoc and Company. The mapping is intentionally boring.

export function docToCompany(id: string, doc: CompanyDoc): Company {
  return Company.reconstitute({ id, ...doc });
}

export function companyToDoc(company: Company): CompanyDoc {
  const { id, ...doc } = company; // exclude id, keep the rest
  return doc;
}

3. Converter

Handle Firestore ↔ CompanyDoc with validation. The converter becomes a safe boundary.

export const companyConverter: FirestoreDataConverter<CompanyDoc> = {
  toFirestore(doc: CompanyDoc) {
    return doc;
  },
  fromFirestore(snapshot) {
    return CompanyDocSchema.parse(snapshot.data());
  },
};

Notice what’s missing: no plainToInstance, no persistence DTO classes, no “hope” that validation is happening.

Repository semantics: make them boring and explicit

Firestore APIs allow too many styles of writing. My repository got much calmer once I made semantics explicit.

• create(company) should fail if the doc already exists
• update(id, params) should enforce domain invariants

And yes: if update uses fetch-modify-persist, that’s a conscious tradeoff: correctness first, optimize later if needed.

Closing thoughts

Firestore is great — but it’s not an ORM, and it doesn’t pretend to be. The weird serialization errors happen when we implicitly expect ORM-like behavior. We pass around class instances assuming some magic layer will serialize them consistently.

My real takeaway wasn’t “don’t use classes.” It was:

• Use classes where behavior lives (domain).
• Use plain objects where data lives (persistence).
• Validate at boundaries.
• Make semantics explicit, or the DB will invent semantics for you.

P.S. In the database, nobody knows you’re an instance.