c# - 尝试序列化 [NotMapped] Entity Framework 属性以供 Breeze 使用

Question

这是一个有点复杂的问题，请耐心等待。我们目前在我们的服务器上使用 Entity Framework 6.1.1，在客户端使用 OData 5.6 和 Breeze JS 1.5.4。简而言之，我们在将模型上的 [NotMapped] 属性序列化为 json 并传递给客户端时遇到了问题。

这是我们的模型:

public class Request 
{
    ...
    public int UserId { get; set; }

    [NotMapped]
    public string UserName {get; set; }
}

因为我们使用的是 OData，而不是通过默认的 JsonMediaTypeFormatter 进行序列化，它通过 OdataMediaTypeFormatter 完全忽略 [NotMapped] 的任何内容 属性。我们可以通过手动将属性添加到 modelBuilder 来解决这个问题。然而，当尝试与 Breeze 集成时，这会成为一个问题，因为它们有自己的自定义 EdmBuilder，必须用于保存可导航属性等内容，而我们不能使用标准的 ODataConventionModelBuilder。这个自定义构建器似乎不允许对模型进行任何级别的控制。是否有可能强制 OData 正确序列化这些属性并保留与 Breeze 不符的元数据？以前有人尝试过类似的东西吗？

边注:我们试图避免在数据库中为这些数据存储或只是创建虚拟列，因为我们需要其中的 5 个属性，但如果我们在这方面投入太多时间，这可能最终会成为我们的行动方针。

提前致谢

Answer 1

在序列化方面，伤害你的是breeze提供的中间EdmBuilder。请参阅:https://github.com/Breeze/breeze.server.labs/blob/master/EdmBuilder.cs

由于EdmBuilder.cs的注释定义的限制

We need the EDM both to define the Web API OData route and as a source of metadata for the Breeze client.  The Web API OData literature recommends the System.Web.Http.OData.Builder.ODataConventionModelBuilder.That component is suffient for route definition but fails as a source of metadata for Breeze because (as of this writing) it neglects to include the foreign key definitions Breeze requires to maintain navigation properties of client-side JavaScript entities.This EDM Builder ask the EF DbContext to supply the metadata which satisfy both route definition and Breeze.You're only getting the metadata the EntityFramework chooses to expose.  This prevents the OData formatters/serializers from including the property - it's not mapped in the model metadata.

You could attempt a solution with a custom serializer, similar to what is represented in this article. Using OData in webapi for properties known only at runtime

A custom serializer would look roughly like this (Note: this DOES NOT work.. continue reading, below...)

public class CustomEntitySerializer : ODataEntityTypeSerializer
{
    public CustomEntitySerializer(ODataSerializerProvider serializerProvider) : base(serializerProvider) {    }

    public override ODataEntry CreateEntry(SelectExpandNode selectExpandNode, EntityInstanceContext entityInstanceContext)
    {
        ODataEntry entry = base.CreateEntry(selectExpandNode, entityInstanceContext);           

        Request item = entityInstanceContext.EntityInstance as Request;
        if (entry != null && item != null)
        {
            // add your "NotMapped" property here.
            entry.Properties = new List<ODataProperty>(entry.Properties) { new ODataProperty { Name = "UserName", Value = item.UserName} };
        }
        return entry;
    }
}

问题在于底层 ODataJsonLightPropertySerializer 在尝试写入时会检查模型是否存在该属性。它调用 Microsoft.Data.OData.WriterValidationUtils 类中的 ValidatePropertyDefined 方法。

internal static IEdmProperty ValidatePropertyDefined(string propertyName, IEdmStructuredType owningStructuredType)

这将使您因运行时异常而失败:

The property 'UserName' does not exist on type 'YourNamespace.Models.Request'. Make sure to only use property names that are defined by the type.","type":"Microsoft.Data.OData.ODataException","stacktrace":" at Microsoft.Data.OData.WriterValidationUtils.ValidatePropertyDefined(String propertyName, IEdmStructuredType owningStructuredType)\r\n at Microsoft.Data.OData.JsonLight.ODataJsonLightPropertySerializer.WriteProperty(ODataProperty property, IEdmStructuredType owningType, Boolean isTopLevel, Boolean allowStreamProperty, DuplicatePropertyNamesChecker duplicatePropertyNamesChecker, ProjectedPropertiesAnnotation projectedProperties

Bottom line is that the property needs to be defined in the model in order to serialize it. You could conceivably rewrite large portions of the serialization layer, but there are lots of internal/static/private/non-virtual bits in the OData framework that make that unpleasant.

A solution is ultimately presented in the way Breeze is forcing you to generate the model, though. Assuming a code-first implementation, you can inject additional model metadata directly into the XmlDocument produced by EntityFramework. Take the method in the Breeze EdmBuilder, with some slight modifications:

static IEdmModel GetCodeFirstEdm<T>(this T dbContext)  where T : DbContext
{
    // create the XmlDoc from the EF metadata
    XmlDocument metadataDocument = new XmlDocument();
    using (var stream = new MemoryStream())
    using (var writer = XmlWriter.Create(stream))
    {
        System.Data.Entity.Infrastructure.EdmxWriter.WriteEdmx(dbContext, writer);
        stream.Position = 0;
        metadataDocument.Load(stream);
    }

    // to support proper xpath queries
    var nsm = new XmlNamespaceManager(metadataDocument.NameTable);
    nsm.AddNamespace("ssdl", "http://schemas.microsoft.com/ado/2009/02/edm/ssdl");
    nsm.AddNamespace("edmx", "http://schemas.microsoft.com/ado/2009/11/edmx");
    nsm.AddNamespace("edm", "http://schemas.microsoft.com/ado/2009/11/edm");

    // find the node we want to work with & add the 1..N property metadata
    var typeElement = metadataDocument.SelectSingleNode("//edmx:Edmx/edmx:Runtime/edmx:ConceptualModels/edm:Schema/edm:EntityType[@Name=\"Request\"]", nsm);

    // effectively, we want to insert this.
    // <Property Name="UserName" Type="String" MaxLength="1000" FixedLength="false" Unicode="true" Nullable="true" />
    var propElement = metadataDocument.CreateElement(null, "Property", "http://schemas.microsoft.com/ado/2009/11/edm");
    propElement.SetAttribute("Name", "UserName");
    propElement.SetAttribute("Type", "String");
    propElement.SetAttribute("FixedLength", "false");
    propElement.SetAttribute("Unicode", "true");
    propElement.SetAttribute("Nullable", "true");

    // append the node to the type element
    typeElement.AppendChild(propElement);

    // now we're going to save the updated xml doc and parse it.
    using (var stream = new MemoryStream())
    {
        metadataDocument.Save(stream);
        stream.Position = 0;
        using (var reader = XmlReader.Create(stream))
        {
            return EdmxReader.Parse(reader);
        }
    }
}

这会将属性放入元数据中以供 OData 层使用，并且不需要任何额外的步骤来促进序列化。但是，您需要注意如何塑造模型元数据，因为任何要求/规范都将反射(reflect)在 Breeze 的客户端验证中。

我已经在 Breeze 提供的 ODataBreezejs 示例中验证了这种方法的 CRUD 操作。 https://github.com/Breeze/breeze.js.samples/tree/master/net/ODataBreezejsSample