The _join field

When using WunderGraph, you might notice that there's a _join field added to all Object Types in your generated GraphQL Schemas. The _join field is a special field that allows you to embed Queries into the response of another Operation. This way, you can "join" multiple heterogeneous APIs into a single response without adding custom resolvers.

Let's look at an example to illustrate this:

query ($code: ID! $capital: String! @internal) {
    # Get the country with the given code
    country: countries_country(code: $code){
        code
        name
        capital @export(as: "capital")
        # "join" a new Query to the returned country to fetch the weather
        weather: _join {
            weather_getCityByName(name: $capital){
                weather {
                    summary {
                        title
                        description
                    }
                    temperature {
                        actual
                    }
                }
            }
        }
    }
}

First, we fetch the country with the given code. Then, we "join" a new Query to the returned country to fetch the weather.

If you look closer, you will notice that the _join simply returns a new Query Type. This way, we're able to make a second embedded Query.

You also see some more concepts in this example, namely the @export and @internal directives. If you want to learn more about them, head over to the reference section on Directives.