The _join field explained
It's quite likely that you've already seen the
_join field in one of the examples. This section will explain how it works and how you can use it.
Let's start by looking at an example. Below is a GraphQL Query that we use in other examples as well. It fetches the countries from a country API, and then "joins" the response with weather information.
How the _join field works
You might have noticed that the
_join field actually exists on all object types. That's specific to WunderGraph, we've simply added it. The return value of the field is the type Query!
In addition to adding it to the Schema, we're also automatically attaching a data source / resolver to all
_join fields. This is just another data source, like REST or GraphQL, but it's a bit different. It's a simple static data source that returns an empty object.
So, what happens at execution time when you're using the
_join field? We're simply embedding another Query root into the enclosing GraphQL Operation. Through the use of the
@export directive, we can export the value of an enclosing field into a variable, making it available as a parameter to the nested Query.