Don't drop empty objects during merge

Author: schmidt-sebastian

packages/firestore/src/api/user_data_converter.ts

@@ -548,6 +548,11 @@ export class UserDataConverter {
         result = result.insert(key, parsedValue);
       }
     });
+    // If we encounter an empty object, we explicitly add it to the update mask
+    // to ensure that the server creates a map entry.
+    if (context.path && result.isEmpty()) {
+      context.fieldMask.push(context.path);
+    }
     return new ObjectValue(result);
   }
 

packages/firestore/test/integration/api/database.test.ts

@@ -188,6 +188,23 @@ apiDescribe('Database', persistence => {
     });
   });
 
+  it.only('can merge empty object', () => {
+    return withTestDoc(persistence, doc => {
+      const initialData = {};
+      const firstMerge = { a: {} };
+      const secondMerge = { b: {} };
+      const finalData = { a: {}, b: {}};
+      return doc
+          .set(initialData)
+          .then(() => doc.set(firstMerge, { mergeFields: ['a'] }))
+          .then(() => doc.set(secondMerge, { merge: true }))
+          .then(() => doc.get())
+          .then(docSnapshot => {
+            expect(docSnapshot.data()).to.be.deep.equal(finalData);
+          });
+    });
+  });
+
   it('can delete field using merge', () => {
     return withTestDoc(persistence, doc => {
       const initialData = {