Context

The main goal of Context is to acquire global data attached to Schema and also data related to the actual query being processed.

Store Data

Inside the Context you can put global data, like environnement variables, db connection pool, whatever you may need in every query.

The data must implement Send and Sync.

You can request the data inside a query by just calling ctx.data::<TypeOfYourData>().

Note that if the return value of resolver function is borrowed from Context, you will need to explicitly state the lifetime of the argument.

The following example shows how to borrow data in Context.

use async_graphql::*;

struct Query;

#[Object]
impl Query {
    async fn borrow_from_context_data<'ctx>(
        &self,
        ctx: &Context<'ctx>
    ) -> Result<&'ctx String> {
        ctx.data::<String>()
    }
}

Schema data

You can put data inside the context at the creation of the schema, it's usefull for data that do not change, like a connection pool.

An instance of how it would be written inside an application:

  let schema = Schema::build(Query::default(), Mutation::default(), EmptySubscription)
    .data(env_struct)
    .data(s3_storage)
    .data(db_core)
    .finish();

Request data

You can put data inside the context at the execution of the request, it's useful for authentication data for instance.

A little example with a warp route:

let graphql_post = warp::post()
  .and(warp::path("graphql"))
  .and(schema_filter)
  .and(a_warp_filter)
  ...
  .and_then( |schema: (Schema<Query, Mutation, Subscriptions>, async_graphql::Request), arg2: ArgType2 ...| async move {
    let (schema, request) = schema;
    let your_auth_data = auth_function_from_headers(headers).await?;
    let response = schema
      .execute(
        request
         .data(your_auth_data)
         .data(something_else)
      ).await;

    Ok(async_graphql_warp::Response::from(response))
  });

Headers

With the Context you can also insert and appends headers.

#[Object]
impl Query {
    async fn greet(&self, ctx: &Context<'_>) -> String {
        // Headers can be inserted using the `http` constants
        let was_in_headers = ctx.insert_http_header(ACCESS_CONTROL_ALLOW_ORIGIN, "*");

        // They can also be inserted using &str
        let was_in_headers = ctx.insert_http_header("Custom-Header", "1234");

        // If multiple headers with the same key are `inserted` then the most recent
        // one overwrites the previous. If you want multiple headers for the same key, use
        // `append_http_header` for subsequent headers
        let was_in_headers = ctx.append_http_header("Custom-Header", "Hello World");

        String::from("Hello world")
    }
}

Selection / LookAhead

Sometimes you want to know what fields are requested in the subquery to optimize the processing of data. You can read fields accross the query with ctx.fields() which will give you a SelectionField which will allow you to navigate accross the fields and subfields.

If you want to perform a search accross the query or the subqueries, you do not have to do this by hand with the SelectionField, you can use the ctx.look_ahead() to perform a selection

use async_graphql::*;

#[derive(SimpleObject)]
struct Detail {
    c: i32,
    d: i32,
}

#[derive(SimpleObject)]
struct MyObj {
    a: i32,
    b: i32,
    detail: Detail,
}

struct Query;

#[Object]
impl Query {
    async fn obj(&self, ctx: &Context<'_>) -> MyObj {
        if ctx.look_ahead().field("a").exists() {
            // This is a query like `obj { a }`
        } else if ctx.look_ahead().field("detail").field("c").exists() {
            // This is a query like `obj { detail { c } }`
        } else {
            // This query doesn't have `a`
        }
        unimplemented!()
    }
}

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