handle absolute / relative paths and full URLs in links

fixes emberjs#1365

absolute paths start with a leading / and will
use only the provided host provided to the adapter

cf. { posts: [{ id: 1, links: { comments: "/posts/1/comments" }}] }

relative paths construct an URL from the record the
relationship is embedded in:

cf. { posts: [{ id: 1, links: { comments: "comments" }}] }

full URLs links will pass through the link transparently:

cf. { posts: [{ id: 1, links: { comments: "http://example.com/posts/1/comments" }}] }

note that all of the above examples are equivalent
to each other.

packages/ember-data/lib/adapters/rest_adapter.js

@@ -247,7 +247,10 @@ DS.RESTAdapter = DS.Adapter.extend({
     @returns Promise
   */
   findHasMany: function(store, record, url) {
-    return this.ajax(this.urlPrefix(url), 'GET');
+    var id   = get(record, 'id'),
+        type = record.constructor.typeKey;
+
+    return this.ajax(this.urlPrefix(url, this.buildURL(type, id)), 'GET');
   },
 
   /**
@@ -280,7 +283,10 @@ DS.RESTAdapter = DS.Adapter.extend({
     @returns Promise
   */
   findBelongsTo: function(store, record, url) {
-    return this.ajax(this.urlPrefix(url), 'GET');
+    var id   = get(record, 'id'),
+        type = record.constructor.typeKey;
+
+    return this.ajax(this.urlPrefix(url, this.buildURL(type, id)), 'GET');
   },
 
   /**
@@ -388,13 +394,26 @@ DS.RESTAdapter = DS.Adapter.extend({
     return url;
   },
 
-  urlPrefix: function(path) {
+  urlPrefix: function(path, parentURL) {
     var host = get(this, 'host'),
         namespace = get(this, 'namespace'),
         url = [];
 
-    if (host) { url.push(host); }
-    if (namespace) { url.push(namespace); }
+    if (path) {
+      // Absolute path
+      if (path.charAt(0) === '/') {
+        if (host) {
+          path = path.slice(1);
+          url.push(host);
+        }
+      // Relative path
+      } else if (!/^http(s)?:\/\//.test(path)) {
+        url.push(parentURL);
+      }
+    } else {
+      if (host) { url.push(host); }
+      if (namespace) { url.push(namespace); }
+    }
 
     if (path) {
       url.push(path);

packages/ember-data/tests/integration/adapter/rest_adapter_test.js

@@ -734,6 +734,60 @@ test('buildURL - with host and namespace', function() {
   }));
 });
 
+test('buildURL - with relative paths in links', function() {
+  adapter.setProperties({
+    host: 'http://example.com',
+    namespace: 'api/v1'
+  });
+  Post.reopen({ comments: DS.hasMany('comment', { async: true }) });
+  Comment.reopen({ post: DS.belongsTo('post') });
+
+  ajaxResponse({ posts: [{ id: 1, links: { comments: 'comments' } }] });
+
+  store.find('post', 1).then(async(function(post) {
+    ajaxResponse({ comments: [{ id: 1 }] });
+    return post.get('comments');
+  })).then(async(function (comments) {
+    equal(passedUrl, "http://example.com/api/v1/posts/1/comments");
+  }));
+});
+
+test('buildURL - with absolute paths in links', function() {
+  adapter.setProperties({
+    host: 'http://example.com',
+    namespace: 'api/v1'
+  });
+  Post.reopen({ comments: DS.hasMany('comment', { async: true }) });
+  Comment.reopen({ post: DS.belongsTo('post') });
+
+  ajaxResponse({ posts: [{ id: 1, links: { comments: '/api/v1/posts/1/comments' } }] });
+
+  store.find('post', 1).then(async(function(post) {
+    ajaxResponse({ comments: [{ id: 1 }] });
+    return post.get('comments');
+  })).then(async(function (comments) {
+    equal(passedUrl, "http://example.com/api/v1/posts/1/comments");
+  }));
+});
+
+test('buildURL - with full URLs in links', function() {
+  adapter.setProperties({
+    host: 'http://example.com',
+    namespace: 'api/v1'
+  });
+  Post.reopen({ comments: DS.hasMany('comment', { async: true }) });
+  Comment.reopen({ post: DS.belongsTo('post') });
+
+  ajaxResponse({ posts: [{ id: 1, links: { comments: 'http://example.com/api/v1/posts/1/comments' } }] });
+
+  store.find('post', 1).then(async(function(post) {
+    ajaxResponse({ comments: [{ id: 1 }] });
+    return post.get('comments');
+  })).then(async(function (comments) {
+    equal(passedUrl, "http://example.com/api/v1/posts/1/comments");
+  }));
+});
+
 test('buildURL - with camelized names', function() {
   adapter.setProperties({
     pathForType: function(type) {