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Tagged Ids

Joist automatically "tags" entity ids, by prefixing them with a per-entity identifier.

For example, assuming the Author entity is configured to use a as it's tag, then returns "a:1" instead of 1:

const a = await em.findOneOrFail(Author, { firstName: "first" });
// Outputs `a:1`

In the database, the column is still an auto-increment integer and has an int value of 1 for this row, Joist just handles automatically adding & removing the a: prefix while loading/saving to the database.

Tag Assignment

For the tag names, when you add a new table, Joist guesses a tag name to use by abbreviating the table name, i.e. book_reviews is br or foo_bar_zazzes is fbz.

If there is a collision, i.e. the br abbreviation is already taken by an existing table in joist-config.json, then Joist will use the full entity name, i.e. bookReview.

The guessed tag name is then stored joist-config.json, where you can easily change it if Joist initially guesses wrong.

However, once you have a given tagged id deployed in production, you should probably never change it (i.e. change the bookReview tag to bkr), because even though Joist internally would immediately start using the new tag value (after the change is deployed), if any other external systems have copies of your ids (like you've stored bookReview:1 in an external/3rd party system), those externally-stored ids will now be incorrect, and Joist will be unload to load them.


There are a few reasons for this feature:

  • Avoiding "Wrong Id" Bugs
  • Easier debugging
  • Convenient GraphQL integration

Avoiding "Wrong Id" Bugs

Knowing the entity type for each id eliminates a class of bugs where ids are passed incorrectly across entity types.

For example, a bug like:

const id =;
// ...lots of lines of code go by...
// Oops, I used an "author id" to find a book...
const book = em.load(Book, id);

Frustratingly, often these "wrong id" bugs will be missed during local testing, because every table only has a few rows of id 1, id 2, so it's easy to have id 1 taken from the authors table and accidentally work when looking it up in the books table.

It's not until production when books id 1,203,345 is accidentally used as an author_id and a "invalid foreign key" constraint fails that we realize we'd try to insert bad data (and may have already been inserting bad data in production up until this point).

Note that Joist's entities also use strongly-typed id types (i.e. returns an AuthorId) to help prevent this with static type checking, but typed ids only prevent "wrong id" bugs that happen internally in the backend code (so, technically within our above example, we should get a compile error that id needs to be a BookId, which is great).

However, tagged ids extends "typed ids"-style protection to API calls, i.e. if a client calls the API for "author a:1" and then makes a subsequent API call that accidentally uses a:1 as a book id, Joist will throw a runtime error that it expected a b:... prefixed id.

Easier Debugging

Seeing tagged ids in console output and error messages makes debugging easier because you immediately know which entity that was for, without having to manually annotate the ids in your logging statements, like with authorId=${...}, or when the id is in JSON payloads.

This seems minor, but in our experience once you've worked with tagged ids in log output, API calls, error messages, etc., you get really attached to the developer experience.

Convenient GraphQL Integration

In GraphQL, there is a dedicated ID type for id fields, e.g. for modeling an type Author { id: ID! } field.

Granted, it is not required to use the ID type, i.e. you can just as well use id: Integer!, but the ID type is encouraged/more idiomatic because it is opaque, meaning it hides the id's implementation details from the client.

I.e., to an external client, it shouldn't really matter if your internal id is "a number" or "a uuid" or "a string", and so having this ID type is how GraphQL represents that opaqueness.

That said, in practice the "opaque" ID type ends up being mapped to strings in actual languages like TypeScript or Go, since a string value can effectively encode/represent other types like a number, or a UUID (albeit with some overhead).

So while Joist is technically GraphQL-agnostic, if you are implementing a GraphQL system (which is what drove Joist's original development), the GraphQL layer already wants "the id is a string", so it is convenient if the Author entity's id is already a string, as then your resolver layer doesn't have to constantly map back/forth from integers to strings for output, and strings to parseInt-d integers for input.

Joist does all of that internally, i.e. "string/number mapping" between the API/entity domain layer and the database columns.

But I'm Not Using GraphQL

Even if you're not using GraphQL, both benefits/rationale of:

  • Id implementations should be opaque to external clients, and
  • Tagged ids prevent "wrong id" bugs

Are applicable to any system, so ideally you could apply the "id is a string" approach to your REST or GRPC or other APIs.

That said, if you have an existing number-based API that you can't change, Joist provides deTagId, deTagIds, and tagId methods to convert to/from tagged ids to the actual number value.

(Also, see the section below for disabling tagged IDs if you'd prefer that.)

Running SQL Queries

When writing raw SQL queries, you can get the numeric value using deTagId

  const query = someKnexQuery();
query.whereIn("", deTagId(getMetadata(Book), bookId));

Note that deTagId accepts the Book entity as its 1st parameter because it still applies the tagged id runtime check, i.e. ensure that bookId starts with b:....

If you need to detag a value without knowing the entity type, you can use unsafeDeTagIds.

Untagged Id Fallback

If you do happen to given Joist untagged ids, it will still work, for example:

const id = "1";
// This will work, the `a:` prefix is not strictly required
const a = await em.load(Author, id);

Disabling Tagged Ids

If you're migrating an existing system to Joist, or just don't want to use tagged ids (although you should try them and see!), you can disable them in the joist-config.json file by setting the idType:

"idType": "untagged-string"

This will change the return value of from "a:1" to just "1".

Note the value is still a string; we've not added support for returning numbers yet, see #368.