service: add etag property to EntityProxy

In OData V2 ETag is located in header or is part of the body in optional
substructure called "__metadata".

https://www.odata.org/documentation/odata-version-2-0/json-format/#7_representing_entries_8

ETag is included in both the header and body of response we check if
they match. If not exception is raised which is our invention and
does not need to be preserved if it causes any problems.

Author: mamiksik

CHANGELOG.md

@@ -11,6 +11,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 - Add a Service wide configuration (e.g. http.update\_method) - Jakub Filak
 - <, <=, >, >= operators on GetEntitySetFilter - Barton Ip
 - Django style filtering - Barton Ip
+- Add etag property to EntityProxy - Martin Miksik
 
 ### Fixed
 - URL encode $filter contents - Barton Ip

pyodata/v2/service.py

@@ -736,20 +736,28 @@ class EntityProxy:
 
     # pylint: disable=too-many-branches,too-many-nested-blocks
 
-    def __init__(self, service, entity_set, entity_type, proprties=None, entity_key=None):
+    def __init__(self, service, entity_set, entity_type, proprties=None, entity_key=None, etag=None):
         self._logger = logging.getLogger(LOGGER_NAME)
         self._service = service
         self._entity_set = entity_set
         self._entity_type = entity_type
         self._key_props = entity_type.key_proprties
         self._cache = dict()
         self._entity_key = entity_key
+        self._etag = etag
 
         self._logger.debug('New entity proxy instance of type %s from properties: %s', entity_type.name, proprties)
 
         # cache values of individual properties if provided
         if proprties is not None:
 
+            etag_body = proprties.get('__metadata', dict()).get('etag', None)
+            if etag is not None and etag_body is not None and etag_body != etag:
+                raise PyODataException('Etag from header does not match the Etag from response body')
+
+            if etag_body is not None:
+                self._etag = etag_body
+
             # first, cache values of direct properties
             for type_proprty in self._entity_type.proprties():
                 if type_proprty.name in proprties:
@@ -921,6 +929,11 @@ def url(self):
 
         return urljoin(service_url, entity_path)
 
+    @property
+    def etag(self):
+        """ETag generated by service"""
+        return self._etag
+
     def equals(self, other):
         """Returns true if the self and the other contains the same data"""
         # pylint: disable=W0212
@@ -1336,8 +1349,9 @@ def get_entity_handler(response):
                                 .format(self._name, response.status_code), response)
 
             entity = response.json()['d']
+            etag = response.headers.get('ETag', None)
 
-            return EntityProxy(self._service, self._entity_set, self._entity_set.entity_type, entity)
+            return EntityProxy(self._service, self._entity_set, self._entity_set.entity_type, entity, etag=etag)
 
         if key is not None and isinstance(key, EntityKey):
             entity_key = key
@@ -1391,8 +1405,9 @@ def create_entity_handler(response):
                                 .format(self._name, response.status_code), response)
 
             entity_props = response.json()['d']
+            etag = response.headers.get('ETag', None)
 
-            return EntityProxy(self._service, self._entity_set, self._entity_set.entity_type, entity_props)
+            return EntityProxy(self._service, self._entity_set, self._entity_set.entity_type, entity_props, etag=etag)
 
         return EntityCreateRequest(self._service.url, self._service.connection, create_entity_handler, self._entity_set,
                                    self.last_segment)

tests/test_service_v2.py

@@ -33,8 +33,14 @@ def test_create_entity(service):
     responses.add(
         responses.POST,
         "{0}/MasterEntities".format(service.url),
-        headers={'Content-type': 'application/json'},
+        headers={
+            'Content-type': 'application/json',
+            'ETag':  'W/\"J0FtZXJpY2FuIEFpcmxpbmVzJw==\"'
+        },
         json={'d': {
+            '__metadata': {
+                'etag': 'W/\"J0FtZXJpY2FuIEFpcmxpbmVzJw==\"',
+            },
             'Key': '12345',
             'Data': 'abcd'
         }},
@@ -44,6 +50,7 @@ def test_create_entity(service):
 
     assert result.Key == '12345'
     assert result.Data == 'abcd'
+    assert result.etag == 'W/\"J0FtZXJpY2FuIEFpcmxpbmVzJw==\"'
 
 
 @responses.activate
@@ -197,11 +204,16 @@ def test_get_entity_property(service):
     responses.add(
         responses.GET,
         "{0}/MasterEntities('12345')".format(service.url),
-        headers={'Content-type': 'application/json'},
+        headers={
+            'ETag': 'W/\"J0FtZXJpY2FuIEFpcmxpbmVzJw==\"',
+            'Content-type': 'application/json',
+        },
         json={'d': {'Key': '12345'}},
         status=200)
 
-    assert service.entity_sets.MasterEntities.get_entity('12345').execute().Key == '12345'
+    result = service.entity_sets.MasterEntities.get_entity('12345').execute()
+    assert result.Key == '12345'
+    assert result.etag == 'W/\"J0FtZXJpY2FuIEFpcmxpbmVzJw==\"'
 
 
 @responses.activate
@@ -2129,6 +2141,30 @@ def test_parsing_of_datetime_before_unix_time(service):
     assert result.Date == datetime.datetime(1945, 5, 8, 19, 0, tzinfo=datetime.timezone.utc)
 
 
+@responses.activate
+def test_mismatched_etags_in_body_and_header(service):
+    """Test creating entity with missmatched etags"""
+
+    responses.add(
+        responses.POST,
+        "{0}/MasterEntities".format(service.url),
+        headers={
+            'Content-type': 'application/json',
+            'ETag':  'W/\"JEF\"'
+        },
+        json={'d': {
+            '__metadata': {
+                'etag': 'W/\"PEF\"',
+            }
+        }},
+        status=201)
+
+    with pytest.raises(PyODataException) as e_info:
+        service.entity_sets.MasterEntities.create_entity().set(**{}).execute()
+
+    assert str(e_info.value) == 'Etag from header does not match the Etag from response body'
+
+
 def test_odata_http_response():
     """Test that ODataHttpResponse is complaint with requests.Reponse"""