GraphQL–HotChocolate - The object type `Object` has to at least define one field in order to be valid.

A colleague contacted me last week asking help with a specific HotChocolate issue they got. He shared the code and the error message with me.

Here is the error he got:

1. The object type `Object` has to at least define one field in order to be valid. (HotChocolate.Types.ObjectType<System.Object>)
      HotChocolate.SchemaException: For more details look at the `Errors` property.
      1. The object type `Object` has to at least define one field in order to be valid. (HotChocolate.Types.ObjectType<System.Object>)
         at HotChocolate.Configuration.TypeInitializer.Initialize()
         at HotChocolate.SchemaBuilder.Setup.InitializeTypes(SchemaBuilder builder, IDescriptorContext context, IReadOnlyList`1 types)
         at HotChocolate.SchemaBuilder.Setup.Create(SchemaBuilder builder, LazySchema lazySchema, IDescriptorContext context)
         at HotChocolate.SchemaBuilder.Create(IDescriptorContext context)
         at HotChocolate.SchemaBuilder.HotChocolate.ISchemaBuilder.Create(IDescriptorContext context)
         at HotChocolate.Execution.RequestExecutorResolver.CreateSchemaAsync(ConfigurationContext context, RequestExecutorSetup setup, RequestExecutorOptions executorOptions, IServiceProvider schemaServices, TypeModuleChangeMonitor typeModuleChangeMonitor, CancellationToken cancellationToken)
         at HotChocolate.Execution.RequestExecutorResolver.CreateSchemaServicesAsync(ConfigurationContext context, RequestExecutorSetup setup, CancellationToken cancellationToken)
         at HotChocolate.Execution.RequestExecutorResolver.GetRequestExecutorNoLockAsync(String schemaName, CancellationToken cancellationToken)
         at HotChocolate.Execution.RequestExecutorResolver.GetRequestExecutorAsync(String schemaName, CancellationToken cancellationToken)
         at HotChocolate.Execution.RequestExecutorProxy.GetRequestExecutorAsync(CancellationToken cancellationToken)
         at HotChocolate.AspNetCore.HttpPostMiddlewareBase.HandleRequestAsync(HttpContext context)
         at HotChocolate.AspNetCore.HttpPostMiddlewareBase.InvokeAsync(HttpContext context)
         at Microsoft.AspNetCore.Builder.EndpointRouteBuilderExtensions.<>c__DisplayClass20_0.<<UseCancellation>b__1>d.MoveNext()
      --- End of stack trace from previous location ---
         at Microsoft.AspNetCore.Routing.EndpointMiddleware.<Invoke>g__AwaitRequestTask|6_0(Endpoint endpoint, Task requestTask, ILogger logger)
         at GrowingActivitiesRegistrations.Api.Middleware.GlobalExceptionHandlingMiddleware.InvokeAsync(HttpContext context, RequestDelegate next) in /GrowingActivitiesRegistrations/src/GrowingActivitiesRegistrations.Api/Middleware/GlobalExceptionHandlingMiddleware.cs:line 21

The error message is coming from the built-in type validation. HotChocolate does multiple checks to validate if your GraphQL schema is correct. One of these checks is to validate if your ObjectType has at least one field defined.

It is exactly this check that fails. As it is doing this check on a class of type ‘Object’, a type that has no fields defined, it makes sense that  we got this error.

So the real question is why is HotChocolate thinking that we have an ObjectType of type ‘Object’?

Let’s have a look at the code!

I was able to pinpoint the issue to the following ObjectType definition:

And more specifically to the ‘Choices’ field. The moment this field was added we got into trouble:

As we didn’t specify an explicit mapping for the Choice type, HotChocolate will infer an ObjectType mapping for us. So let’s have a look at the Choice class:

Do you notice the problem? The Choice class inherits from ValueObject that exposes a GetAtomicValues function that returns an IEnumerable<Object>. That explains where the error is coming from.

To fix it we can create an explicit ObjectType for Choice where we ignore the GetAtomicValues function:

We also have to update the Choices field to return the correct type: