Scalars

Cynic supports all the built in GraphQL scalars by default. If you want to query a field of one of these types add a field of the corresponding Rust type to your QueryFragment struct.

  • String fields in GraphQL should be String fields in Rust.
  • Int fields in GraphQL should be i32 in Rust.
  • Boolean fields in GraphQL map to bool in Rust.
  • ID fields in GraphQL map to the cynic::Id type in Rust.

Custom Scalars

impl_scalar!

GraphQL allows a schema to define it's own scalars - cynic also supports these.

If you have an existing type (including 3rd party types) that has a serde::Serialize impl and want to use it as a Scalar, you can use impl_scalar! to register it as a Scalar. For example to register chrono::DateTime<chrono::Utc> as a DateTime scalar:


#![allow(unused_variables)]
fn main() {
type DateTime = chrono::DateTime<chrono::Utc>;
impl_scalar!(DateTime, schema::DateTime);
}

This DateTime type alias can now be used anywhere that the schema expects a DateTime. Note that the type alias is currently required due to limitations in some of the cynic macros (though this may not always be the case).

#[derive(Scalar)]

You can also derive Scalar on any newtype structs:


#![allow(unused_variables)]
fn main() {
#[derive(cynic::Scalar, serde::Serialize)]
#[cynic(schema_module = "schema")]
struct MyScalar(String);
}

This MyScalar type can now be used anywhere the schema expects a MyScalar.

Any types that derive cynic::Scalar must also derive (or otherwise implement) serde::Serialize. You can change the inner type that's used to deserialize the scalar by changing the type inside the struct.

Note that this derive only works on newtype structs - for any more complex datatype you'll have to implement cynic::Scalar yourself.

Struct Attributes

A Scalar derive can be configured with several attributes on the struct itself:

  • graphql_type = "AType" can be provided if the type of the struct differs from the type of and tells cynic the name of the Scalar in the schema. This defaults to the name of the struct if not provided.
  • schema_module tells cynic where to find the query module - that is a module that has called the use_schema! macro. This is required but can also be provided by nesting the QueryFragment inside a query module.