Fix contained navigation paths when keys are missing

src/Microsoft.OData.Core/ODataWriterCore.cs

@@ -2785,9 +2785,14 @@ private void EnterScope(WriterState newState, ODataItem item)
                             {
                                 case EdmNavigationSourceKind.ContainedEntitySet:
                                     // Containment cannot be written alone without odata uri.
+                                    bool pathBuiltFromResourceId = false;
                                     if (!odataPath.Any())
                                     {
-                                        throw new ODataException(SRResources.ODataWriterCore_PathInODataUriMustBeSetWhenWritingContainedElement);
+                                        pathBuiltFromResourceId = this.TryBuildPathFromResourceId(out odataPath);
+                                        if (odataPath == null || !odataPath.Any())
+                                        {
+                                            throw new ODataException(SRResources.ODataWriterCore_PathInODataUriMustBeSetWhenWritingContainedElement);
+                                        }
                                     }
 
                                     ODataPath newPath = null;
@@ -2796,6 +2801,11 @@ private void EnterScope(WriterState newState, ODataItem item)
                                         // if there's no key, for example in a complex property, just use the existing odata path.
                                         newPath = odataPath;
                                     }
+                                    else if (pathBuiltFromResourceId && !odataPath.IsCollection())
+                                    {
+                                        // A resource ID that targets a single resource already represents its complete path.
+                                        newPath = odataPath;
+                                    }
                                     else
                                     {
                                         // If there's only non-key properties selected in the request URI, then We don't have the key values to calcuate the key segment.
@@ -2808,7 +2818,8 @@ private void EnterScope(WriterState newState, ODataItem item)
                                         }
                                         else
                                         {
-                                            newPath = null;
+                                            ODataPath resourcePath;
+                                            newPath = this.TryBuildPathFromResourceId(out resourcePath) ? resourcePath : null;
                                         }
                                     }
 
@@ -2903,6 +2914,33 @@ private bool TryBuildKeySegment(out KeySegment keySegment)
             return true;
         }
 
+        private bool TryBuildPathFromResourceId(out ODataPath resourcePath)
+        {
+            ODataResourceBase resource = this.CurrentScope.Item as ODataResourceBase;
+            Uri resourceId = resource != null && resource.HasNonComputedId ? resource.NonComputedId : null;
+            Uri serviceRoot = this.CurrentScope.ODataUri.ServiceRoot;
+            if (resourceId == null || serviceRoot == null)
+            {
+                resourcePath = null;
+                return false;
+            }
+
+            try
+            {
+                resourcePath = new ODataUriParser(
+                    this.outputContext.Model,
+                    serviceRoot,
+                    resourceId,
+                    this.outputContext.Container).ParsePath();
+                return resourcePath.Any();
+            }
+            catch (ODataException)
+            {
+                resourcePath = null;
+                return false;
+            }
+        }
+
         private IList<KeyValuePair<string, object>> GetKeyProperties(IEdmEntityType currentEntityType, bool throwIfFail)
         {
             ODataResourceBase resource = this.CurrentScope.Item as ODataResourceBase;