Fix union queries not working in @requires (#3347)

* remove seamingly unneccessary type check

* implement logic to separate union/interface queries properly into their own 'field'

* add specific check to exclude union queries from typechecks

* add schema example to show it working

* fix feedback/linter issues

* include union @requires example in computed_requires

* go generate ./...

---------

Co-authored-by: Rick Bijkerk <rickbijkerk@bol.com>

plugin/federation/federation.go

@@ -279,10 +279,13 @@ func (f *Federation) GenerateCode(data *codegen.Data) error {
 				typeString := strings.Split(obj.Type.String(), ".")
 				requiresImports[strings.Join(typeString[:len(typeString)-1], ".")] = true
 
+				if containsUnionField(reqField) {
+					continue
+				}
+
 				cgField := reqField.Field.TypeReference(obj, data.Objects)
 				reqField.Type = cgField.TypeReference
 			}
-
 			// add type info to entity
 			e.Type = obj.Type
 		}
@@ -329,6 +332,15 @@ func (f *Federation) GenerateCode(data *codegen.Data) error {
 	})
 }
 
+func containsUnionField(reqField *Requires) bool {
+	for _, requireFields := range reqField.Field {
+		if strings.HasPrefix(requireFields, "... on") {
+			return true
+		}
+	}
+	return false
+}
+
 // Fill in types for key fields
 func populateKeyFieldTypes(
 	resolver *EntityResolver,

plugin/federation/fieldset/fieldset.go

@@ -133,8 +133,22 @@ func (f Field) LastIndex() int {
 // parseUnnestedKeyFieldSet // handles simple case where none of the fields are nested.
 func parseUnnestedKeyFieldSet(raw string, prefix []string) Set {
 	ret := Set{}
+	unionField := false
 
 	for _, s := range strings.Fields(raw) {
+		if s == "..." {
+			continue
+		}
+		if s == "on" {
+			unionField = true
+			continue
+		}
+
+		if unionField {
+			s = "... on " + s
+			unionField = false
+		}
+
 		next := append(prefix[0:len(prefix):len(prefix)], s) //nolint:gocritic // set cap=len in order to force slice reallocation
 		ret = append(ret, next)
 	}