[FIX] web: properly handle onchanges after discard

When an onchange is resolved after a discard, it caused a traceback. We
now make sure that it does not happen.  Note: I think that the test is
quite neat.

addons/web/static/src/js/views/basic/basic_model.js

@@ -1049,6 +1049,13 @@ var BasicModel = AbstractModel.extend({
                 args: [idList, currentData, fields, onchange_spec, context],
             })
             .then(function (result) {
+                if (!record._changes) {
+                    // if the _changes key does not exist anymore, it means that
+                    // it was removed by discarding the changes after the rpc
+                    // to onchange. So, in that case, the proper response is to
+                    // ignore the onchange.
+                    return;
+                }
                 if (result.warning) {
                     self.trigger_up('warning', {
                         message: result.warning.message,

addons/web/static/test/views/form_tests.js

@@ -2058,5 +2058,57 @@ QUnit.module('Views', {
             });
         }
     });
+
+    QUnit.test('onchanges that complete after discarding', function (assert) {
+        assert.expect(4);
+
+        var def1 = $.Deferred();
+
+        this.data.partner.onchanges = {
+            foo: function (obj) {
+                obj.int_field = obj.foo.length + 1000;
+            },
+        };
+        var form = createView({
+            View: FormView,
+            model: 'partner',
+            data: this.data,
+            arch: '<form>' +
+                    '<group><field name="foo"/><field name="int_field"/></group>' +
+                '</form>',
+            res_id: 2,
+            mockRPC: function (route, args) {
+                var result = this._super.apply(this, arguments);
+                if (args.method === 'onchange') {
+                    assert.step('onchange is done');
+                    return def1.then(function () {
+                        return result;
+                    });
+                }
+                return result;
+            },
+        });
+
+        // go into edit mode
+        assert.strictEqual(form.$('span[name="foo"]').text(), "blip",
+            "field foo should be displayed to initial value");
+        form.$buttons.find('.o_form_button_edit').click();
+
+        // edit a value
+        form.$('input').first().val("1234").trigger('input');
+
+        // discard changes
+        form.$buttons.find('.o_form_button_cancel').click();
+        $('.modal .modal-footer .btn-primary').click();
+        assert.strictEqual(form.$('span[name="foo"]').text(), "blip",
+            "field foo should still be displayed to initial value");
+
+        // complete the onchange
+        def1.resolve();
+        assert.strictEqual(form.$('span[name="foo"]').text(), "blip",
+            "field foo should still be displayed to initial value");
+
+        form.destroy();
+    });
 });
 });