Field Selection

Field selection occurs when you have a compound type (an object or interface type) and you select a set of sub fields from that type.

For example given the following query :

query {
  user(userId : "xyz") {
    name
    age
    weight
    friends {
      name
    }
  }
}

The field selection set of the user field is name, age, weight, friends and friends/name

Knowing the field selection set can help make DataFetchers more efficient. For example in the above query imagine that the user field is backed by an SQL database system. The data fetcher could look ahead into the field selection set and use different queries because it knows the caller wants friend information as well as user information.

graphql.schema.DataFetchingFieldSelectionSet is used to represent this field selection set. It gives you maps of the fields and their graphql.schema.GraphQLFieldDefinitions and argument values.

DataFetcher smartUserDF = new DataFetcher() {
    @Override
    public Object get(DataFetchingEnvironment env) {
        String userId = env.getArgument("userId");

        DataFetchingFieldSelectionSet selectionSet = env.getSelectionSet();
        if (selectionSet.contains("friends/*")) {
            return getUserAndTheirFriends(userId);
        } else {
            return getUser(userId);
        }
    }
};

A glob path matching system is used for addressing fields in the selection. Its based on java.nio.file.FileSystem#getPathMatcher as an implementation.

This will allow you to use *, ** and ? as special matching characters such that invoice/customer* would match an invoice field with child fields that start with customer. Each level of field is separated by / just like a file system path.

There are methods that allow you to get more detailed information about the fields in the selection set. For example if you are using Relay often you want to know what fields have been request in the Connection section of the query.

So given a query like:

query {
  users(first:10)  {
    edges {
      node {
        name
        age
        weight
        friends {
          name
        }
      }
    }
  }
}

you can write code that gets the details of each specific field that matches a glob.

DataFetchingFieldSelectionSet selectionSet = env.getSelectionSet();
List<SelectedField> nodeFields = selectionSet.getFields("edges/nodes/*");
nodeFields.forEach(selectedField -> {
    System.out.println(selectedField.getName());
    System.out.println(selectedField.getFieldDefinition().getType());

    DataFetchingFieldSelectionSet innerSelectionSet = selectedField.getSelectionSet();
    // this forms a tree of selection and you can get very fancy with it
}