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copied!<p>The distinction between the custom <code>EndpointsAliasProperty</code>s and one of the data properties you defined is how they are used. They are all used to create a <code>protorpc</code> message, and that message is then converted into an <code>EndpointsModel</code> with your custom data in it. <strong>THIS</strong> is where the magic happens.</p> <p>Breaking it down into steps:</p> <h2>1. You specify your data</h2> <pre><code>from google.appengine.ext import ndb from endpoints_proto_datastore.ndb import EndpointsModel class MyModel(EndpointsModel): my_attr = ndb.StringProperty() </code></pre> <h2>2. You pick your fields for your method</h2> <pre><code>class MyApi(...): @MyModel.method(request_fields=('id', 'my_attr'), ...) def my_method(self, my_model_entity): ... </code></pre> <h2>3. A <code>protorpc</code> message class is defined from your fields</h2> <pre><code>>>> request_message_class = MyModel.ProtoModel(fields=('id', 'my_attr')) >>> request_message_class <class '.MyModelProto_id_my_attr'> >>> for field in request_message_class.all_fields(): ... print field.name, ':', field.variant ... id : INT64 my_attr : STRING </code></pre> <p>This happens <a href="https://github.com/GoogleCloudPlatform/endpoints-proto-datastore/blob/511c8ca87d7548b90e9966495e9ebffd5eecab6e/endpoints_proto_datastore/ndb/model.py#L1262">every time</a> a request is handled by a method decorated with <code>@MyModel.method</code>.</p> <h2>4. A request comes in your application and a message is created</h2> <p>Using the <code>protorpc</code> message class, a message instance is parsed from the JSON which gets passed along to your Endpoints <a href="http://en.wikipedia.org/wiki/Service_provider_interface">SPI</a> (which is created by <code>endpoints.api_server</code>).</p> <p>When the request comes in to your <code>protorpc.remote.Service</code> it is <a href="https://code.google.com/p/google-protorpc/source/browse/python/protorpc/wsgi/service.py?r=60e8f9831a0cefb30ea2024f4ae96f71b1b606b4#147">decoded</a>:</p> <pre><code>>>> from protorpc import remote >>> protocols = remote.Protocols.get_default() >>> json_protocol = protocols.lookup_by_content_type('application/json') >>> request_message = json_protocol.decode_message( ... request_message_class, ... '{"id": 123, "my_attr": "some-string"}' ... ) >>> request_message <MyModelProto_id_my_attr id: 123 my_attr: u'some-string'> </code></pre> <h2>5. The <code>protorpc</code> message is <a href="https://github.com/GoogleCloudPlatform/endpoints-proto-datastore/blob/511c8ca87d7548b90e9966495e9ebffd5eecab6e/endpoints_proto_datastore/ndb/model.py#L1316">cast into a datastore model</a></h2> <pre><code>entity = MyModel.FromMessage(request_message) </code></pre> <p><strong>THIS</strong> is the step you really care about. The <code>FromMessage</code> class method (also provided as part of <code>EndpointsModel</code>) loops through all the fields</p> <pre><code>for field in sorted(request_message_class.all_fields(), key=lambda field: field.number): </code></pre> <p>and for each field with a value set, turns the value into something to be added to the entity and separates based on whether the property is an <code>EndpointsAliasProperty</code> or not:</p> <pre><code> if isinstance(value_property, EndpointsAliasProperty): alias_args.append((local_name, to_add)) else: entity_kwargs[local_name] = to_add </code></pre> <p>After completing this loop, we have an ordered list <code>alias_args</code> of all key, value pairs and a dictionary <code>entity_kwargs</code> of the data attributes parsed from the message.</p> <p>Using these, first a simple entity <a href="https://github.com/GoogleCloudPlatform/endpoints-proto-datastore/blob/511c8ca87d7548b90e9966495e9ebffd5eecab6e/endpoints_proto_datastore/ndb/model.py#L1162">is created</a></p> <pre><code>entity = MyModel(**entity_kwargs) </code></pre> <p>and then each of the alias property values are set <strong>in order</strong>:</p> <pre><code>for name, value in alias_args: setattr(entity, name, value) </code></pre> <hr> <p>The extended behavior happens in <code>setattr(entity, name, value)</code>. Since <code>EndpointsAliasProperty</code> is a subclass of <a href="http://docs.python.org/2/library/functions.html#property"><code>property</code></a>, it is a descriptor and it has a <code>setter</code> which can perform some custom behavior <strong>beyond</strong> simply setting a value.</p> <p>For example, the <code>id</code> property is <a href="https://github.com/GoogleCloudPlatform/endpoints-proto-datastore/blob/511c8ca87d7548b90e9966495e9ebffd5eecab6e/endpoints_proto_datastore/ndb/model.py#L764">defined with</a>:</p> <pre><code>@EndpointsAliasProperty(setter=IdSet, property_type=messages.IntegerField) def id(self): </code></pre> <p>and the <a href="https://github.com/GoogleCloudPlatform/endpoints-proto-datastore/blob/511c8ca87d7548b90e9966495e9ebffd5eecab6e/endpoints_proto_datastore/ndb/model.py#L745"><code>setter</code></a> performs operations beyond simply setting data:</p> <pre><code>def IdSet(self, value): self.UpdateFromKey(ndb.Key(self.__class__, value)) </code></pre> <p>This particular method attempts to retrieve the entity stored in the datastore using the <code>id</code> and patch in any values from the datastore that were not included in the entity parsed from the request.</p> <hr> <p>If you wanted to do this for a field like <code>my_attr</code>, you would need to construct a custom query which could retrieve the item with that unique <code>my_attr</code> value (or fail if not exactly one such entity exists).</p> <p>This is problematic and you'd be better off using a unique field like the key or ID used to store the entity in the datastore.</p> <p>The <a href="http://endpoints-proto-datastore.appspot.com/examples/keys_with_ancestors.html">keys with ancestors</a> sample gives a great example of creating your own custom properties.</p> <p>If you <strong>REALLY</strong> insist on using <code>my_attr</code> to retrieve an entity, you could do so using a different property name (since <code>my_attr</code> is already used for the data property) such as <code>fromMyAttr</code>:</p> <pre><code>class MyModel(EndpointsModel): def MyAttrSet(self, value): ... @EndpointsAliasProperty(setter=MyAttrSet) def fromMyAttr(self): ... </code></pre> <p>Here, the <code>MyAttrSet</code> instance method would form the query:</p> <pre><code> def MyAttrSet(self, value): query = MyModel.query(MyModel.my_attr == value) results = query.fetch(2) </code></pre> <p>reject results that aren't unique for <code>my_attr</code>:</p> <pre><code> if len(results) == 0: raise endpoints.NotFoundException('Not found.') if len(results) == 2: raise endpoints.BadRequestException('Colliding results.') </code></pre> <p>and copy over the values for the already stored entity if we do find a unique one:</p> <pre><code> matching_entity = results[0] self._CopyFromEntity(matching_entity) self._from_datastore = True </code></pre>

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