async-graphql/docs/en/src/define_complex_object.md

Object

Different from SimpleObject, Object must have a resolver defined for each field in its impl.

A resolver function has to be asynchronous. The first argument has to be &self, the second is an optional Context and it is followed by field arguments.

The resolvers is used to get the value of the field. For example, you can query a database and return the result. The return type of the function is the type of the field. You can also return a async_graphql::FieldResult to return an error if it occurs. The error message will then be sent to query result.

You may need access to global data in your query, for example a database connection pool. When creating your Schema, you can use SchemaBuilder::data to configure the global data, and Context::data to configure Context data. The following value_from_db function shows how to retrieve a database connection from Context.

use async_graphql::*;

struct MyObject {
    value: i32,
}

#[Object]
impl MyObject {
    async fn value(&self) -> String {
        self.value.to_string()
    }

    async fn value_from_db(
        &self,
        ctx: &Context<'_>,
        #[arg(desc = "Id of object")] id: i64
    ) -> FieldResult<String> {
        let conn = ctx.data::<DbPool>()?.take();
        Ok(conn.query_something(id)?.name)
    }
}