Add a better error message when findRecord returns an array

Closes #3950

addon/system/store/finders.js

@@ -35,6 +35,7 @@ export function _find(adapter, store, typeClass, id, internalModel, options) {
     Ember.assert("You made a `find` request for a " + typeClass.typeClassKey + " with id " + id + ", but the adapter's response did not have any data", payloadIsNotBlank(adapterPayload));
     return store._adapterRun(function() {
       var payload = normalizeResponseHelper(serializer, store, typeClass, adapterPayload, id, 'findRecord');
+      Ember.assert('Ember Data expected the primary data returned from a `findRecord` response to be an object but instead it found an array.', !Array.isArray(payload.data));
       //TODO Optimize
       var record = store.push(payload);
       return record._internalModel;

tests/integration/store-test.js

@@ -571,3 +571,17 @@ test("Store should accept a null value for `data`", function(assert) {
     });
   });
 });
+
+test('store#findRecord that returns an array should assert', assert => {
+  initializeStore(DS.JSONAPIAdapter.extend({
+    findRecord: function() {
+      return { data: [] };
+    }
+  }));
+
+  assert.expectAssertion(function() {
+    run(function() {
+      store.findRecord('car', 1);
+    });
+  }, /expected the primary data returned from a `findRecord` response to be an object but instead it found an array/);
+});