Question

我正在尝试创建一个软件包，该软件包将使用JSON文件中的数据帮助种子数据库，这些JSON文件可以手动创建或从旧数据库中提取。目前，我已经创建了一个通用类SeedCreator<T>，该类从给定文件（实体名称加.json）中检索JSON，并将其反序列化为给定类型的对象。这部分工作正常。

为了使此过程尽可能动态，我通过标识名称空间为Entities的所有类，在项目中的实体上使用反射。这样，我就遍历检索到的List并检查JSON文件是否存在。如果可以，则将路径和通用类型传递给类SeedsCreator。在运行Add-Migration时进行调试时，数据会从JSON文件中恢复，但是返回变量modelBuilder后，我得到了错误The seed entity for entity type 'Table1' cannot be added because there was no value provided for the required property 'Id'.

如果我手动输入以下内容，则效果很好。

modelBuilder.Entity(typeof(Table1)).HasData(data);

任何帮助将不胜感激。特别是如果我是盲人并且做过非常简单而愚蠢的事情。

public class Seeds
{
    public ModelBuilder CreateSeeds(ModelBuilder modelBuilder)
    {
        var entities = (from t in Assembly.GetExecutingAssembly().GetTypes()
                       where t.Namespace != null && (t.IsClass && t.Namespace.Contains("Entities"))
                       select t).ToList();

        foreach (var type in entities)
        {
            if (File.Exists("./Seeds/" + type.Name + ".json"))
            {
                Type[] typeArr = { type };
                var seeds = typeof(SeedCreator<>).MakeGenericType(typeArr);
                var activatedSeeds = Activator.CreateInstance(seeds);
                var data = seeds.GetMethod("GetSeeds")?.Invoke(activatedSeeds, new object[] { "./Seeds/" + type.Name + ".json" });
                modelBuilder.Entity(type).HasData(data);
            }
        }

        return modelBuilder;
    }
}

public class SeedCreator<T>
{
    public List<T> GetSeeds(string jsonPath)
    {
        using (var sr = new StreamReader(jsonPath))
            return JsonConvert.DeserializeObject<List<T>>(sr.ReadToEnd());
    }
}

public class Table1
{
    public int Id { get; set; }
}

在DbContext内部

using (var dataSeed = new Seeds()) modelBuilder = dataSeed.CreateSeeds(modelBuilder);

示例JSON文件（Table1.json）

[
  {
    "id": 1
  },
  {
    "id": 2
  }
]

堆栈跟踪

System.InvalidOperationException: The seed entity for entity type 'Table1' cannot be added because there was no value provided for the required property 'Id'. at Microsoft.EntityFrameworkCore.Infrastructure.ModelValidator.ValidateData(IModel model) at Microsoft.EntityFrameworkCore.Infrastructure.ModelValidator.Validate(IModel model) at Microsoft.EntityFrameworkCore.Infrastructure.RelationalModelValidator.Validate(IModel model) at Microsoft.EntityFrameworkCore.Internal.SqlServerModelValidator.Validate(IModel model) at Microsoft.EntityFrameworkCore.Metadata.Conventions.Internal.ValidatingConvention.Apply(InternalModelBuilder modelBuilder) at Microsoft.EntityFrameworkCore.Metadata.Conventions.Internal.ConventionDispatcher.ImmediateConventionScope.OnModelBuilt(InternalModelBuilder modelBuilder) at Microsoft.EntityFrameworkCore.ModelBuilder.FinalizeModel() at System.Lazy`1.ViaFactory(LazyThreadSafetyMode mode) at System.Lazy`1.ExecutionAndPublication(LazyHelper executionAndPublication, Boolean useDefaultConstructor) at System.Lazy`1.CreateValue() at Microsoft.EntityFrameworkCore.Internal.DbContextServices.CreateModel() at Microsoft.EntityFrameworkCore.Internal.DbContextServices.get_Model() at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitScoped(ScopedCallSite scopedCallSite, ServiceProviderEngineScope scope) at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitConstructor(ConstructorCallSite constructorCallSite, ServiceProviderEngineScope scope) at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitScoped(ScopedCallSite scopedCallSite, ServiceProviderEngineScope scope) at Microsoft.Extensions.DependencyInjection.ServiceProviderServiceExtensions.GetRequiredService(IServiceProvider provider, Type serviceType) at Microsoft.Extensions.DependencyInjection.ServiceProviderServiceExtensions.GetRequiredService[T](IServiceProvider provider) at Microsoft.EntityFrameworkCore.DbContext.get_DbContextDependencies() at Microsoft.EntityFrameworkCore.DbContext.get_InternalServiceProvider() at Microsoft.EntityFrameworkCore.Internal.InternalAccessorExtensions.GetService[TService](IInfrastructure`1 accessor) at Microsoft.EntityFrameworkCore.Design.Internal.DbContextOperations.CreateContext(Func`1 factory) at Microsoft.EntityFrameworkCore.Design.Internal.DbContextOperations.CreateContext(String contextType) at Microsoft.EntityFrameworkCore.Design.Internal.MigrationsOperations.AddMigration(String name, String outputDir, String contextType) at Microsoft.EntityFrameworkCore.Design.OperationExecutor.AddMigrationImpl(String name, String outputDir, String contextType) at Microsoft.EntityFrameworkCore.Design.OperationExecutor.OperationBase.<>c__DisplayClass3_0`1.<Execute>b__0() at Microsoft.EntityFrameworkCore.Design.OperationExecutor.OperationBase.Execute(Action action)

Answer 1

您正陷入典型的params object[]陷阱。

HasData方法有2个重载-一个重载为params object[] data，另一个重载为IEnumerable<object> data。由于进行了反射调用，因此data变量的类型为object。因此，您正在调用包含您的object[]的单个项目data的第一个重载。

如果您想知道为什么异常消息告诉您没有为属性提供任何值，则说明很简单。 HasData不需要传递的对象与要播种的实体具有相同的类型。这是为了允许指定影子属性，该属性在实体类中不存在，但对于数据种子来说是必需的。因此，它允许您传递包含所有实体属性的任何匿名或具体类型。

因此，它试图反映所传递对象的实际类型并找到属性Id。由于您所传递的单个对象的实际类型是List<TEntity>，因此它当然没有Id属性，因此没有异常消息。

话虽如此，解决方法当然是调用正确的HasData重载（使用IEnumerable<object> data）：

modelBuilder.Entity(type).HasData((IEnumerable<object>)data);

种子抛出“无值提供”异常的添加迁移

1 个答案: