Skip printing empty fragments

Summary: Fragments can be empty if all non-generated fields are skipped due to `include` or `skip` directives - these shouldn't be printed.
Closes #634

Reviewed By: yungsters

Differential Revision: D2701245

fb-gh-sync-id: dc4cbf9176e46a9ecd23311fb31c40bcc9d3e385

src/__forks__/traversal/__tests__/printRelayOSSQuery-test.js

@@ -288,6 +288,30 @@ describe('printRelayOSSQuery', () => {
       `);
       expect(variables).toEqual({});
     });
+
+    it('omits empty inline fragments', () => {
+      var fragment = getNode(Relay.QL`
+        fragment on Viewer {
+          actor {
+            id
+          }
+          ... on Viewer {
+            actor @include(if: $false) {
+              name
+            }
+          }
+        }
+      `, {false: false});
+      var {text} = printRelayOSSQuery(fragment);
+      expect(text).toEqualPrintedQuery(`
+        fragment PrintRelayOSSQuery on Viewer {
+          actor {
+            id,
+            __typename
+          }
+        }
+      `);
+    });
   });
 
   describe('fields', () => {

src/traversal/printRelayOSSQuery.js

@@ -160,7 +160,10 @@ function printFragment(
 function printInlineFragment(
   node: RelayQuery.Fragment,
   printerState: PrinterState
-): string {
+): ?string {
+  if (!node.getChildren().length) {
+    return null;
+  }
   var fragmentID = node.getFragmentID();
   var {fragmentMap} = printerState;
   if (!(fragmentID in fragmentMap)) {
@@ -210,20 +213,26 @@ function printChildren(
   node: RelayQuery.Node,
   printerState: PrinterState
 ): string {
-  var children = node.getChildren().map(node => {
+  let children;
+  node.getChildren().forEach(node => {
     if (node instanceof RelayQuery.Field) {
-      return printField(node, printerState);
+      children = children || [];
+      children.push(printField(node, printerState));
     } else {
       invariant(
         node instanceof RelayQuery.Fragment,
         'printRelayOSSQuery(): expected child node to be a `Field` or ' +
         '`Fragment`, got `%s`.',
         node.constructor.name
       );
-      return printInlineFragment(node, printerState);
+      const printedFragment = printInlineFragment(node, printerState);
+      if (printedFragment) {
+        children = children || [];
+        children.push(printedFragment);
+      }
     }
   });
-  if (!children.length) {
+  if (!children) {
     return '';
   }
   return '{' + children.join(',') + '}';