Variables

GraphQL queries can declare variables that can be passed in, allowing you to set the values of arguments without specifying those values directly in your query. You can use this to pass values from the rest of your program into cynic.

You can declare a set of variables by making a struct and deriving QueryVariables on it:

#![allow(unused)]
fn main() {
#[derive(cynic::QueryVariables)]
struct FilmVariables {
    id: Option<cynic::Id>,
}
}

The struct above declares a single variable named id of Option<cynic::Id> (or ID in GraphQL terms).

Using QueryVariables

To use variables in a query you need to tell cynic which QueryVariables struct is in scope. You do this by providing a a variables parameter to the QueryFragment derive. This allows you to provide any of the variables in your QueryVariables struct to any arguments in this fragment. For example:

#![allow(unused)]
fn main() {
#[derive(cynic::QueryFragment, Debug)]
#[cynic(
    graphql_type = "Root",
    variables = "FilmVariables"
)]
struct FilmQuery {
    #[arguments(id: $id)]
    film: Option<Film>,
}
}

This example uses our FilmVariables at the root of the query to specify which film we want to fetch.

It's also possible to pass variables down to lower levels of the query using the same technique. Though it's worth noting that all the QueryFragments from the Root to the point that requires arguments must define the same variables in their cynic attribute. If no nested QueryFragments require any variables then it's OK to omit variables.

Struct Attributes

QueryVariables can be configured via attributes at the struct level:

• rename_all="camelCase" tells cynic to rename all the rust field names with a particular rule to match their GraphQL counterparts. If not provided this defaults to camelCase to be consistent with GraphQL conventions.

Field Attributes

Each field can also have attributes:

• graphql_type="path" can be used to override the GraphQL type that cynic detects for the field.
• rename="someFieldName" can be used to map a field to a completely different GraphQL field name.
• skip_serializing_if="path" can be used on optional fields to skip serializing them. By default an Option field will be sent as null to servers, but if you provide skip_serializing_if="Option::is_none" then the field will not be provided at all.