This note is in Hasura.RemoteSchema.SchemaCache.Permission. It is referenced at:
Remote schema argument presets are a way to inject values from static values or
from session variables during execution. Presets can be set using the preset
directive, the preset directive is defined in the following manner:
scalar PresetValue
directive @preset(
value: PresetValue
) on INPUT_FIELD_DEFINITION | ARGUMENT_DEFINITION
When a preset directive is defined on an input type, the input type is removed from the schema and the value is injected by the graphql-engine during the execution.
There are two types of preset:
Static preset is used to preset a static GraphQL value which will be injected during the execution of the query. Static presets can be specified on all types of input types i.e scalars, enums and input objects and lists of these types. The preset value (if specified) will be validated against the type it’s provided. For example:
users(user_id: Int @preset(value: {user_id: 1}))
The above example will throw an error because the preset is attempting to preset an input object value for a scalar (Int) type.
Session variable preset is used to inject value from a session variable into the
graphql query during the execution. If the value argument of the preset directive
is in the format of the session variable i.e. x-hasura-* it will be treated as a
session variable preset. During the execution of a query, which has a session variable
preset set, the session variable’s will be looked up and the value will be constructed
into a GraphQL variable. Check out resolveRemoteVariable for more details about how
the session variable presets get resolved.
At the time of writing this note, session variable presets can only be specified at named types and only for scalar and enum types. This is done because currently there’s no good way to specify array or object values through session variables.