fix polymorphic belongsTo with include

modelTo should not be expected

Author: Resonance1584

lib/serializer.js

@@ -27,7 +27,7 @@ function defaultSerialize (options, cb) {
    */
   if (options.requestedIncludes) {
     try {
-      handleIncludes(resultData, options.requestedIncludes, options.relationships);
+      handleIncludes(resultData, options.requestedIncludes, options.relationships, options.app);
     } catch (err) {
       cb(err);
     }
@@ -313,7 +313,7 @@ function makeLinks (links, item) {
  * @throws {Error}
  * @return {undefined}
  */
-function handleIncludes (resp, includes, relations) {
+function handleIncludes (resp, includes, relations, app) {
   var resources = _.isArray(resp.data) ? resp.data : [resp.data];
 
   if (typeof includes === 'string') {
@@ -328,7 +328,13 @@ function handleIncludes (resp, includes, relations) {
     return includes.map(function (include) {
       var relation = relations[include];
       var propertyKey = relation.keyFrom;
-      var plural = utils.pluralForModel(relation.modelTo);
+      var plural = '';
+      if (relation.polymorphic) {
+        var discriminator = resource.attributes[relation.polymorphic.discriminator];
+        plural = utils.pluralForModel(app.models[discriminator]);
+      } else {
+        plural = utils.pluralForModel(relation.modelTo);
+      }
       var embeds = [];
 
       // If relation is belongsTo then pk and fk are the other way around

test/belongsToPolymorphic.test.js

@@ -63,5 +63,17 @@ describe('loopback json api belongsTo polymorphic relationships', function () {
           done();
         });
     });
+
+    it('should return the Post that this file belongs to when included flag is present', function (done) {
+      request(app).get('/files/1?include=parent')
+        .end(function (err, res) {
+          expect(err).to.equal(null);
+          expect(res.body.errors).to.equal(undefined);
+          expect(res.body.included).to.be.an('array');
+          expect(res.body.included[0].type).to.equal('posts');
+          expect(res.body.included[0].id).to.equal('1');
+          done();
+        });
+    });
   });
 });