@identifier

Every projection needs an identifier to uniquely identify the data. This ID is generated automatically by the system and is used to identify the data in the projection. You might want to duplicate this field with the generated ID using another field name. Declare the @identifier directive to mark a field as the identifier of the projection.

Example

type User @upsertOn(...) {
    userId: ID! @identifier
}

In this case, the @identifier directive marks the userId field as a second identifying field of the User projection.