set the entity to it's default value is present in the mapper and if the entity is not provided in the response.

runtime/ms-rest/lib/serialization.js

@@ -416,7 +416,7 @@ function serializeDateTypes(typeName, value, objectName) {
  */
 exports.deserialize = function (mapper, responseBody, objectName) {
   if (responseBody === null || responseBody === undefined) {
-    if (mapper && mapper.isConstant) {
+    if (mapper && mapper.defaultValue) {
       responseBody = mapper.defaultValue;
     } else {
       return responseBody;
@@ -577,7 +577,7 @@ function deserializeCompositeType(mapper, responseBody, objectName) {
         if (Array.isArray(responseBody[key]) && modelProps[key].serializedName === '') {
           propertyInstance = responseBody[key];
           instance = exports.deserialize.call(this, propertyMapper, propertyInstance, propertyObjectName);
-        } else if ((propertyInstance !== null && propertyInstance !== undefined) || (propertyMapper && propertyMapper.isConstant)) {
+        } else if ((propertyInstance !== null && propertyInstance !== undefined) || (propertyMapper && propertyMapper.defaultValue)) {
           serializedValue = exports.deserialize.call(this, propertyMapper, propertyInstance, propertyObjectName);
           instance[key] = serializedValue;
         }

runtime/ms-rest/test/serializationTests.js

@@ -1120,6 +1120,7 @@ describe('msrest', function () {
       result.id.should.equal(5);
       result.status.should.equal(true);
       result.eyeColor.should.equal('brown');
+      result.favoriteFood.should.equal('bones');
       result.odatalocation.should.equal('westus');
       result.color.should.equal('red');
       result.city.should.equal('Seattle');