Parse nested objects (#666)

Summary:
In the current version nested objects are not handled correctly.

For example, we want to retrieve Ads for the campaign using only one request. With this fix, we'll be able to make requests like:

```
campaigns = account.get_campaigns(fields=['name,ads{id,name,status}'])
campaign = campaigns[0]
```

In this example nested Ads objects will be available as

```
campaign.get('ads')
```

Pull Request resolved: #666

Reviewed By: mengxuanzhangz

Differential Revision: D54815751

Pulled By: stcheng

fbshipit-source-id: f23299bc168c33e485830ce500d8c8841cfbaf02

Author: aluminiumgeek

facebook_business/adobjects/objectparser.py

@@ -9,6 +9,7 @@
 )
 from facebook_business.adobjects.abstractobject import AbstractObject
 
+
 class ObjectParser:
     """
     Parser for API response
@@ -39,30 +40,48 @@ def __init__(
         self._custom_parse_method = custom_parse_method
         self._api = api
 
-    def parse_single(self, response):
+    def parse_single(self, response, override_target_class=None):
         if self._custom_parse_method is not None:
             return self._custom_parse_method(response, self._api)
 
+        from .ad import Ad
+        from .adpreview import AdPreview
+        from .adset import AdSet
+        from .campaign import Campaign
+
         data = response
         if 'data' in response and isinstance(response['data'], dict):
             data = response['data']
         elif 'images' in response and not isinstance(data['images'], list):
             _, data = data['images'].popitem()
-        if 'campaigns' in data:
-            _, data = data['campaigns'].popitem()
-        elif 'adsets' in data:
-            _, data = data['adsets'].popitem()
-        elif 'ads' in data:
-            _, data = data['ads'].popitem()
+
+        subfields = (
+            ('campaigns', Campaign),
+            ('adsets', AdSet),
+            ('ads', Ad),
+            ('previews', AdPreview),
+        )
+        for subfield, _class in subfields:
+            if subfield not in data:
+                continue
+
+            data[subfield] = [
+                self.parse_single(
+                    item, override_target_class=_class
+                ) for item in data[subfield]['data']
+            ]
+
         if 'success' in data:
             del data['success']
 
+        target_class = override_target_class or self._target_class
+
         if self._reuse_object is not None:
             self._reuse_object._set_data(data)
             return self._reuse_object
         elif self._target_class is not None:
             return AbstractObject.create_object(self._api, data,
-                                                self._target_class)
+                                                target_class)
         else:
             raise FacebookBadObjectError(
                 'Must specify either target class calling object' +