Tags an implementing object with the value its parent interface’s @discriminate(on:) column must hold for a row to belong to this subtype. Pairs with @discriminate for single-table polymorphism. Without @discriminate on the interface there is nothing for the value to match against; without @discriminator on each implementer the row mapper has no way to assign rows.

SDL signature

directive @discriminator(value: String!) on OBJECT

Parameters

Name Type Default Description

value

String!

(required)

The literal database value of the discriminator column that places a row in this subtype. Compared verbatim against the column at row-mapping time; case-sensitive.

Canonical example

From the example schema’s Content family:

interface Content @table(name: "content") @discriminate(on: "CONTENT_TYPE") {
    contentId: Int!    @field(name: "CONTENT_ID")
    title:     String! @field(name: "TITLE")
}

type FilmContent implements Content @table(name: "content") @discriminator(value: "FILM") {
    contentId: Int!       @field(name: "CONTENT_ID")
    title:     String!    @field(name: "TITLE")
    length:    Int        @field(name: "LENGTH")
    rating:    MpaaRating @reference(path: [{key: "content_film_id_fkey"}]) @field(name: "RATING")
}

type ShortContent implements Content @table(name: "content") @discriminator(value: "SHORT") {
    contentId:   Int!    @field(name: "CONTENT_ID")
    title:       String! @field(name: "TITLE")
    description: String  @field(name: "SHORT_DESCRIPTION")
}

Each implementer also carries @table(name: "content") (the same table as the interface) and @discriminator(value: …​). At row-mapping time the fetcher reads content_type; rows where content_type = 'FILM' map to FilmContent, rows where content_type = 'SHORT' map to ShortContent. Rows with any other discriminator value fail to map and surface as a runtime error.

Constraints

  • @discriminator is only meaningful on objects that implement an interface decorated with @discriminate. The build rejects it on objects whose implemented interfaces have no discriminator column.

  • The implementer’s @table must match the interface’s @table. Mixing tables breaks single-table polymorphism.

  • value is matched verbatim against the database column; quoting and case must match the stored value exactly.

  • Each implementer of a single @discriminate interface should declare a distinct value. Two implementers sharing the same value collapse to one branch and the second is unreachable.

  • The directive applies only to OBJECT; interfaces and unions cannot carry @discriminator.

See also

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