[OpenAPI] Get description with [FromQuery]

Get the description from the associated object's property when `[FromQuery]` is applied to a property of an object used as a `[FromQuery]` parameter.

Resolves #61297.

Author: martincostello

src/OpenApi/src/Services/OpenApiDocumentService.cs

@@ -494,11 +494,22 @@ private static bool IsRequired(ApiParameterDescription parameter)
     }
 
     // Apply [Description] attributes on the parameter to the top-level OpenApiParameter object and not the schema.
-    private static string? GetParameterDescriptionFromAttribute(ApiParameterDescription parameter) =>
-        parameter.ParameterDescriptor is IParameterInfoParameterDescriptor { ParameterInfo: { } parameterInfo } &&
-        parameterInfo.GetCustomAttributes().OfType<DescriptionAttribute>().LastOrDefault() is { } descriptionAttribute ?
-            descriptionAttribute.Description :
-            null;
+    private static string? GetParameterDescriptionFromAttribute(ApiParameterDescription parameter)
+    {
+        if (parameter.ParameterDescriptor is IParameterInfoParameterDescriptor { ParameterInfo: { } parameterInfo } &&
+            parameterInfo.GetCustomAttributes<DescriptionAttribute>().LastOrDefault() is { } parameterDescription)
+        {
+            return parameterDescription.Description;
+        }
+
+        if (parameter.ModelMetadata is Mvc.ModelBinding.Metadata.DefaultModelMetadata { Attributes.PropertyAttributes.Count: > 0 } metadata &&
+            metadata.Attributes.PropertyAttributes.OfType<DescriptionAttribute>().LastOrDefault() is { } propertyDescription)
+        {
+            return propertyDescription.Description;
+        }
+
+        return null;
+    }
 
     private async Task<OpenApiRequestBody?> GetRequestBodyAsync(OpenApiDocument document, ApiDescription description, IServiceProvider scopedServiceProvider, IOpenApiSchemaTransformer[] schemaTransformers, CancellationToken cancellationToken)
     {

src/OpenApi/test/Microsoft.AspNetCore.OpenApi.Tests/Services/OpenApiDocumentServiceTestsBase.cs

@@ -229,7 +229,7 @@ public ControllerActionDescriptor CreateActionDescriptor(string methodName = nul
 
         action.AttributeRouteInfo = new()
         {
-            Template = action.MethodInfo.GetCustomAttribute<RouteAttribute>()?.Template,
+            Template = action.MethodInfo.GetCustomAttribute<RouteAttribute>()?.Template ?? string.Empty,
             Name = action.MethodInfo.GetCustomAttribute<RouteAttribute>()?.Name,
             Order = action.MethodInfo.GetCustomAttribute<RouteAttribute>()?.Order ?? 0,
         };

src/OpenApi/test/Microsoft.AspNetCore.OpenApi.Tests/Services/OpenApiSchemaService/OpenApiSchemaService.ParameterSchemas.cs

@@ -509,6 +509,39 @@ await VerifyOpenApiDocument(builder, document =>
         });
     }
 
+    [Fact]
+    public async Task GetOpenApiParameters_HandlesAsParametersParametersWithDescriptionAttribute()
+    {
+        // Arrange
+        var builder = CreateBuilder();
+
+        // Act
+        builder.MapGet("/api", ([AsParameters] FromQueryModel model) => { });
+
+        // Assert
+        await VerifyOpenApiDocument(builder, document =>
+        {
+            var operation = document.Paths["/api"].Operations[HttpMethod.Get];
+            var parameter = Assert.Single(operation.Parameters);
+            Assert.Equal("The ID of the entity", parameter.Description);
+        });
+    }
+
+    [Fact]
+    public async Task GetOpenApiParameters_HandlesFromQueryParametersWithDescriptionAttribute()
+    {
+        // Arrange
+        var actionDescriptor = CreateActionDescriptor(nameof(TestFromQueryController.GetWithFromQueryDto), typeof(TestFromQueryController));
+
+        // Assert
+        await VerifyOpenApiDocument(actionDescriptor, document =>
+        {
+            var operation = document.Paths["/"].Operations[HttpMethod.Get];
+            var parameter = Assert.Single(operation.Parameters);
+            Assert.Equal("The ID of the entity", parameter.Description);
+        });
+    }
+
     [Route("/api/{id}/{date}")]
     private void AcceptsParametersInModel(RouteParamsContainer model) { }
 
@@ -809,4 +842,23 @@ public override void Write(Utf8JsonWriter writer, EnumArrayType value, JsonSeria
             writer.WriteEndObject();
         }
     }
+
+    [ApiController]
+    [Route("[controller]/[action]")]
+    private class TestFromQueryController : ControllerBase
+    {
+        [HttpGet]
+        public Task<IActionResult> GetWithFromQueryDto([FromQuery] FromQueryModel query)
+        {
+            return Task.FromResult<IActionResult>(Ok());
+        }
+    }
+
+    [Description("A query model.")]
+    private record FromQueryModel
+    {
+        [Description("The ID of the entity")]
+        [FromQuery(Name = "id")]
+        public int Id { get; set; }
+    }
 }