Forbid subscription queries with more than 1 root field

As per spec. See error message in the commit.

Author: tomhoule

graphql_query_derive/src/schema.rs

@@ -137,6 +137,13 @@ impl Schema {
                         let prefix = format!("RUST_{}", prefix);
                         let selection = Selection::from(&q.selection_set);
 
+                        if selection.0.len() > 0 {
+                            Err(format_err!(
+                                "{}",
+                                ::constants::MULTIPLE_SUBSCRIPTION_FIELDS_ERROR
+                            ))?
+                        }
+
                         definitions.extend(
                             definition.field_impls_for_selection(&context, &selection, &prefix)?,
                         );

tests/subscription/subscription_invalid_query.graphql

@@ -0,0 +1,8 @@
+subscription InvalidSubscription($filter: String) {
+  newDogs {
+    name
+  }
+  dogBirthdays(filter: $filter) {
+    name
+  }
+}

tests/subscription/subscription_schema.graphql

@@ -0,0 +1,32 @@
+schema {
+  query: SimpleQuery
+  mutation: SimpleMutation
+  subscription: SimpleSubscription
+}
+
+type SimpleQuery {
+  dogByName(name: String): Dog
+}
+
+type SimpleMutation {
+  petDog(dogName: String): Dog
+}
+
+type SimpleSubscription {
+  newDogs: Dog
+  dogBirthdays(filter: String): DogBirthday
+}
+
+type DogBirthday {
+  date: String
+  age: Int
+  treats: [String]
+}
+
+type Dog {
+  name: String!
+  """
+  Always returns true
+  """
+  isGoodDog: Boolean!
+}

tests/subscriptions.rs

@@ -0,0 +1,20 @@
+#[macro_use]
+extern crate graphql_client;
+#[macro_use]
+extern crate serde_derive;
+extern crate serde;
+extern crate serde_json;
+
+// If you comment this out, it will not compile because the query is not valid. We need to investigate how we can make this a real test.
+//
+// #[derive(GraphQLQuery)]
+// #[graphql(
+//     schema_path = "tests/subscription/subscription_schema.graphql",
+//     query_path = "tests/subscription/subscription_invalid_query.graphql"
+// )]
+// struct SubscriptionInvalidQuery;
+
+#[test]
+fn subscriptions_work() {
+    unimplemented!("subscriptions test");
+}