Cynic

Cynic is a GraphQL query builder & data mapper for Rust.

See the README on GitHub for more details.

Overview

To get started with Cynic you'll need a GraphQL schema for the API you wish to query. The examples will be using the star wars API.

Generating a Query DSL

Once you've got your schema installed locally, you'll need to use the query_dsl macro to generate a query_dsl for your schema:

mod query_dsl {
cynic::query_dsl!("../examples/examples/starwars.schema.graphql");
}

This macro generates a few things:

1. Some structs to represent the Input types the underlying schema. You may need to use these to build mutations or as parameters to queries.
2. Definitons of all the Enums in the provided schema. You'll need these if you want to use any enum types.
3. Type safe selection set functions. These can be used to build up a query manually, though it's usually easier to use the QueryFragment derive functionality explained below. Hopefully you'll not need to use these directly too often.

Though using macros to generate these is convenient, it does leave a lot of code to the imagination. You can get a glimpse of the things this defines by running cargo doc --document-private-items and having a look in the query_dsl module. It's not ideal, but at least provides some visibility into the various enum types.

Creating QueryFragments

Now that you have a query_dsl defined, you can start building some queries. Cynic lets you do this by deriving QueryFragment for a struct. For example, if we wanted to know what director title & director a Star Wars film had, we could define this QueryFragment:

# mod query_dsl {
#   cynic::query_dsl!("../examples/examples/starwars.schema.graphql");
# }

#[derive(cynic::QueryFragment)]
#[cynic(
schema_path = "../examples/examples/starwars.schema.graphql",
query_module = "query_dsl",
graphql_type = "Film"
)]
struct Film {
title: Option<String>,
director: Option<String>
}

// This `Film` struct can now be used as the type of a field on any other
// `QueryFragment` struct and cynic will know how to turn that into a GraphQL
// query, and populate the `Film` struct from the response.

// For example, if we wanted to know the Director for a particular film:

#[derive(cynic::QueryFragment)]
#[cynic(
schema_path = "../examples/examples/starwars.schema.graphql",
query_module = "query_dsl",
graphql_type = "Root"
)]
struct FilmDirectorQuery {
// Here we use the `#[cynic_arguments()]` attribute on the `film` field to provide a
// hard coded film ID to look up.  Though useful for demonstration, hard coded
// arguments like this aren't much use in reality.  For more details on providing
// runtime arguments please see below.
#[cynic_arguments(id = Some("ZmlsbXM6MQ==".into()))]
film: Option<Film>,
}

// You can then build a `cynic::Query` from this fragment
use cynic::QueryFragment;
let query = cynic::Query::new(FilmDirectorQuery::fragment(()));

query above implements serde::Serialize so can be used with any HTTP client. For example, with reqwest:

let response = reqwest::blocking::Client::new()
.post("a_url")
.json(&query)
.send()?;
let result = query.decode_response(response.json()?)?;

After this code has run, result will be an instance of FilmDirectorQuery with the film populated appropriately.

Dynamic Query Arguments

The query above was useful for demonstration, but you'll usually want to be able to provide parameters to your query. To do this, you should define a struct that contains all of the parameters you want to provide:

# mod query_dsl {
#   cynic::query_dsl!("../examples/examples/starwars.schema.graphql");
# }

# #[derive(cynic::QueryFragment)]
# #[cynic(
#     schema_path = "../examples/examples/starwars.schema.graphql",
#    query_module = "query_dsl",
#    graphql_type = "Film"
# )]
# struct Film {
#    title: Option<String>,
#    director: Option<String>
# }
// Deriving `FragmentArguments` allows this struct to be used as arguments to a
// `QueryFragment` fragment, whether it represents part of a query or a whole query.
#[derive(cynic::FragmentArguments, Clone)]
struct FilmArguments {
id: Option<cynic::Id>
}

// You can now define a query to use these arguments on.  For example, to make
// `FilmDirectorQuery` a bit more dynamic:
#[derive(cynic::QueryFragment)]
#[cynic(
schema_path = "../examples/examples/starwars.schema.graphql",
query_module = "query_dsl",
graphql_type = "Root",
// By adding the `argument_struct` parameter to our `QueryFragment` we've made a variable
// named `args` avaiable for use in the `cynic_arguments` attribute.
argument_struct = "FilmArguments"
)]
struct FilmDirectorQueryWithArgs {
// Here we use `args`, which we've declared above to be an instance of `FilmArguments`
#[cynic_arguments(id = args.id.clone())]
film: Option<Film>,
}

// Then we can build a query using this new struct;
use cynic::QueryFragment;
let query = cynic::Query::new(
FilmDirectorQueryWithArgs::fragment(
FilmArguments{ id: Some("ZmlsbXM6MQ==".into()) }
)
);