Introduced new GetContentTypeDescriptor() method

On the surface, the new `GetContentTypeDescriptor()` appears to just be a convenience method for checking the `GetContentTypeDescriptors() to determine if it contains the `ContentTypeDescriptor` associated with the current `Topic` and, if so, returning it.

The `GetContentTypeDescriptor()` method does a lot more than this, however. If the `ContentTypeDescriptor` _cannot_ be found, then it attempts to locate the `Root:Configuration:ContentTypes` topic within the in-memory topic graph using the new `GetTopicByUniqueKey()` extension method (f2cb109). If found, it will then merge that tree with the `GetContentTypeDescriptors()` cache by using the new overload (7a5a243). In this way, it is able to fall back from persisted `ContentTypeDescriptor`s loaded from the database to potentially rely on new—but not yet persisted—`ContentTypeDescriptor`s available in the current graph.

As part of this, the new `GetContentTypeDescriptor()` method is used for the content type lookups in the local `GetAttributes()` and `GetUnmatchedAttributes()` methods, which are in turn used by the concrete `Save()` implementation.

This provides a reactive solution for the discovering in-memory `ContentTypeDescriptor` on `Save()` (#18). In practice, we'll generally prefer a proactive solution by updating these as `ContentTypeDescriptor`s when they're saved, instead of discovering them only when `Save()` is unable to find them. That will be addressed in a future commit.

For now, however, this provides a fallback implementation, and will be necessary for supporting more complex scenarios such as bootstrapping the database, or otherwise importing complex topic graphs using e.g. the **OnTopic Data Exchange** library.

Includes two unit tests for validating the functionality.

Author: JeremyCaney

OnTopic.TestDoubles/StubTopicRepository.cs

@@ -184,6 +184,13 @@ public IEnumerable<AttributeValue> GetAttributesProxy(Topic topic, bool? isExten
     public ContentTypeDescriptorCollection GetContentTypeDescriptorsProxy(ContentTypeDescriptor topicGraph) =>
       base.GetContentTypeDescriptors(topicGraph);
 
+    /*==========================================================================================================================
+    | METHOD: GET CONTENT TYPE DESCRIPTOR (PROXY)
+    \-------------------------------------------------------------------------------------------------------------------------*/
+    /// <inheritdoc cref="TopicRepositoryBase.GetContentTypeDescriptor(Topic)" />
+    public ContentTypeDescriptor? GetContentTypeDescriptorProxy(Topic sourceTopic) =>
+      base.GetContentTypeDescriptor(sourceTopic);
+
     /*==========================================================================================================================
     | METHOD: CREATE FAKE DATA
     \-------------------------------------------------------------------------------------------------------------------------*/

OnTopic.Tests/TopicRepositoryBaseTest.cs

@@ -168,5 +168,37 @@ public void GetContentTypeDescriptors_WithTopicGraph_ReturnsMergedContentTypes()
 
     }
 
+    /*==========================================================================================================================
+    | TEST: GET CONTENT TYPE DESCRIPTOR: GET VALID CONTENT TYPE: RETURNS CONTENT TYPE
+    \-------------------------------------------------------------------------------------------------------------------------*/
+    /// <summary>
+    ///   Attempts to retrieve a specific <see cref="ContentTypeDescriptor"/> by its <see cref="Topic.Key"/>.
+    /// </summary>
+    [TestMethod]
+    public void GetContentTypeDescriptor_GetValidContentType_ReturnsContentType() {
+
+      var topic                 = TopicFactory.Create("Test", "Page");
+      var contentType           = _topicRepository.GetContentTypeDescriptorProxy(topic);
+
+      Assert.IsNotNull(contentType);
+
+    }
+
+    /*==========================================================================================================================
+    | TEST: GET CONTENT TYPE DESCRIPTOR: GET INVALID CONTENT TYPE: RETURNS NULL
+    \-------------------------------------------------------------------------------------------------------------------------*/
+    /// <summary>
+    ///   Attempts to retrieve an invalid <see cref="ContentTypeDescriptor"/>.
+    /// </summary>
+    [TestMethod]
+    public void GetContentTypeDescriptor_GetInvalidContentType_ReturnsNull() {
+
+      var topic                 = TopicFactory.Create("Test", "InvalidContentType");
+      var contentType           = _topicRepository.GetContentTypeDescriptorProxy(topic);
+
+      Assert.IsNull(contentType);
+
+    }
+
   } //Class
 } //Namespace

OnTopic/Repositories/TopicRepositoryBase.cs

@@ -139,6 +139,54 @@ topic is ContentTypeDescriptor contentTypeDescriptor &&
 
     }
 
+    /*==========================================================================================================================
+    | GET CONTENT TYPE DESCRIPTOR
+    \-------------------------------------------------------------------------------------------------------------------------*/
+    /// <summary>
+    ///   Attempts to identify the <see cref="ContentTypeDescriptor"/> for the provided <paramref name="sourceTopic"/>.
+    /// </summary>
+    /// <remarks>
+    ///   The <see cref="GetContentTypeDescriptor(Topic)"/> method will attempt to get the <see cref="ContentTypeDescriptor"/>
+    ///   from the <see cref="GetContentTypeDescriptors()"/> method using the <paramref name="sourceTopic"/>'s <see
+    ///   cref="Topic.ContentType"/>. If that can't be found, however, then it will instead look in the <paramref
+    ///   name="sourceTopic"/>'s topic graph to see if the <see cref="ContentTypeDescriptor"/> can be found there. This is
+    ///   useful for cases where new topic graphs are being imported and a new <see cref="Topic"/> references a new <see
+    ///   cref="ContentTypeDescriptor"/> prior to it having been saved. In this case, that new version will be added to the
+    ///   locally cached collection used by <see cref="GetContentTypeDescriptors()"/>.
+    /// </remarks>
+    /// <param name="sourceTopic"></param>
+    /// <returns></returns>
+    protected ContentTypeDescriptor? GetContentTypeDescriptor(Topic sourceTopic) {
+      /*------------------------------------------------------------------------------------------------------------------------
+      | Validate parameters
+      \-----------------------------------------------------------------------------------------------------------------------*/
+      Contract.Requires(sourceTopic, nameof(sourceTopic));
+
+      /*------------------------------------------------------------------------------------------------------------------------
+      | Retrieve content type
+      \-----------------------------------------------------------------------------------------------------------------------*/
+      var contentType           = sourceTopic.ContentType;
+      var contentTypes          = GetContentTypeDescriptors();
+      var contentTypeDescriptor = contentTypes.Contains(contentType)? contentTypes[contentType] : null;
+
+      if (contentTypeDescriptor != null) {
+        return contentTypeDescriptor;
+      }
+
+      /*------------------------------------------------------------------------------------------------------------------------
+      | Retrieve content type
+      \-----------------------------------------------------------------------------------------------------------------------*/
+      if (
+        sourceTopic.GetByUniqueKey("Root:Configuration:ContentTypes") is ContentTypeDescriptor sourceContentTypes &&
+        !contentTypes.Contains(sourceContentTypes)
+      ) {
+        contentTypes            = GetContentTypeDescriptors(sourceContentTypes);
+      }
+
+      return contentTypes.Contains(contentType)? contentTypes[contentType] : null;
+
+    }
+
     /*==========================================================================================================================
     | METHOD: LOAD
     \-------------------------------------------------------------------------------------------------------------------------*/
@@ -383,7 +431,7 @@ protected IEnumerable<AttributeValue> GetAttributes(Topic topic, bool? isExtende
       /*------------------------------------------------------------------------------------------------------------------------
       | Get associated content type descriptor
       \-----------------------------------------------------------------------------------------------------------------------*/
-      var contentType           = GetContentTypeDescriptors()[topic.Attributes.GetValue("ContentType", "Page")?? "Page"];
+      var contentType           = GetContentTypeDescriptor(topic);
 
       Contract.Assume(
         contentType,
@@ -432,11 +480,11 @@ protected IEnumerable<AttributeDescriptor> GetUnmatchedAttributes(Topic topic) {
       /*------------------------------------------------------------------------------------------------------------------------
       | Get associated content type descriptor
       \-----------------------------------------------------------------------------------------------------------------------*/
-      var contentType           = GetContentTypeDescriptors()[topic.Attributes.GetValue("ContentType", "Page")?? "Page"];
+      var contentType           = GetContentTypeDescriptor(topic);
 
       Contract.Assume(
         contentType,
-        "The Topics repository or database does not contain a ContentTypeDescriptor for the Page content type."
+        $"The Topics repository or database does not contain a ContentTypeDescriptor for the {topic.ContentType} content type."
       );
 
       /*------------------------------------------------------------------------------------------------------------------------