Treat empty strings in ids the same as null or undefined

packages/ember-data/lib/system/coerce-id.js

@@ -5,5 +5,5 @@
 // ID into the URL, and if we later try to deserialize that URL and find the
 // corresponding record, we will not know if it is a string or a number.
 export default function coerceId(id) {
-  return id == null ? null : id+'';
+  return id == null || id === '' ? null : id+'';
 }

packages/ember-data/tests/integration/client-id-generation-test.js

@@ -1,5 +1,5 @@
 var get = Ember.get;
-var Post, Comment, env;
+var Post, Comment, Misc, env;
 var run = Ember.run;
 
 module("integration/client_id_generation - Client-side ID Generation", {
@@ -12,9 +12,14 @@ module("integration/client_id_generation - Client-side ID Generation", {
       comments: DS.hasMany('comment')
     });
 
+    Misc = DS.Model.extend({
+      foo: DS.attr('string')
+    });
+
     env = setupStore({
       post: Post,
-      comment: Comment
+      comment: Comment,
+      misc: Misc
     });
   },
 
@@ -60,3 +65,35 @@ test("If an adapter implements the `generateIdForRecord` method, the store shoul
     post.save();
   });
 });
+
+test("empty string and undefined ids should coerce to null", function() {
+  expect(6);
+  var comment, post;
+  var idCount = 0;
+  var ids = [undefined, ''];
+  env.adapter.generateIdForRecord = function(passedStore, record) {
+    equal(env.store, passedStore, "store is the first parameter");
+
+    return ids[idCount++];
+  };
+
+  env.adapter.createRecord = function(store, type, record) {
+    equal(typeof get(record, 'id'), 'object', 'correct type');
+    return Ember.RSVP.resolve();
+  };
+
+  run(function() {
+    comment = env.store.createRecord('misc');
+    post = env.store.createRecord('misc');
+  });
+
+  equal(get(comment, 'id'), null, "comment is assigned id 'null'");
+  equal(get(post, 'id'), null, "post is assigned id 'null'");
+
+  // Despite client-generated IDs, calling commit() on the store should still
+  // invoke the adapter's `createRecord` method.
+  run(function() {
+    comment.save();
+    post.save();
+  });
+});