Support __typename for unions

See http://graphql.org/learn/queries/#meta-fields.

Fixes https://github.com/graphql-rust/juniper/issues/112.

juniper/src/executor_tests/interfaces_unions.rs

@@ -204,6 +204,7 @@ mod union {
         let doc = r"
           {
             pets {
+              __typename
               ... on Dog {
                 name
                 woofs
@@ -228,10 +229,12 @@ mod union {
             Value::object(vec![
                 ("pets", Value::list(vec![
                     Value::object(vec![
+                        ("__typename", Value::string("Dog")),
                         ("name", Value::string("Odie")),
                         ("woofs", Value::boolean(true)),
                     ].into_iter().collect()),
                     Value::object(vec![
+                        ("__typename", Value::string("Cat")),
                         ("name", Value::string("Garfield")),
                         ("meows", Value::boolean(false)),
                     ].into_iter().collect()),

juniper/src/validation/rules/fields_on_correct_type.rs

@@ -1,4 +1,5 @@
 use ast::Field;
+use schema::meta::MetaType;
 use validation::{ValidatorContext, Visitor};
 use parser::Spanning;
 
@@ -16,6 +17,19 @@ impl<'a> Visitor<'a> for FieldsOnCorrectType {
                 let type_name = parent_type.name().unwrap_or("<unknown>");
 
                 if parent_type.field_by_name(field_name.item).is_none() {
+
+                    match *parent_type {
+                        MetaType::Union(..) => {
+                            // You can query for `__typename` on a union,
+                            // but it isn't a field on the union...it is
+                            // instead on the resulting object returned.
+                            if field_name.item == "__typename" {
+                                return;
+                            }
+                        }
+                        _ => {}
+                    }
+
                     context.report_error(
                         &error_message(field_name.item, type_name),
                         &[field_name.start.clone()],
@@ -318,6 +332,24 @@ mod tests {
         );
     }
 
+    #[test]
+    fn typename_on_union() {
+        expect_passes_rule(
+            factory,
+            r#"
+          fragment objectFieldSelection on Pet {
+            __typename
+            ... on Dog {
+              name
+            }
+            ... on Cat {
+              name
+            }
+          }
+        "#,
+        );
+    }
+
     #[test]
     fn valid_field_in_inline_fragment() {
         expect_passes_rule(