[QUESTION] why dotnet-aspnet-codegenerator generating a new DbContext

[doggy8088]

Steps to reproduce:

1. Create a Database in SQL Server Express LocalDB: https://gist.github.com/doggy8088/2a2f7075d49b3814d19513426ede3549
2. dotnet new webapi -n api1 && cd api1
3. dotnet add package Microsoft.VisualStudio.Web.CodeGeneration.Design
4. dotnet add package Microsoft.EntityFrameworkCore.Design
5. dotnet add package Microsoft.EntityFrameworkCore.SqlServer
6. dotnet tool update -g dotnet-ef
7. dotnet ef dbcontext scaffold "Server=(localdb)\MSSQLLocalDB;Database=ContosoUniversity;Trusted_Connection=True;MultipleActiveResultSets=true" Microsoft.EntityFrameworkCore.SqlServer -o Models -f
8. dotnet aspnet-codegenerator controller -name CoursesController -async -api -m Course -dc myContext -outDir Controllers

Building project ...
Finding the generator 'controller'...
Running the generator 'controller'...
Generating a new DbContext class 'myContext'
Attempting to compile the application in memory with the added DbContext.
Attempting to figure out the EntityFramework metadata for the model and DbContext: 'Course'
The entity type 'CourseInstructor' requires a primary key to be defined. If you intended to use a keyless entity type call 'HasNoKey()'. StackTrace:
   at Microsoft.EntityFrameworkCore.Infrastructure.ModelValidator.ValidateNonNullPrimaryKeys(IModel model, IDiagnosticsLogger`1 logger)
   at Microsoft.EntityFrameworkCore.Infrastructure.ModelValidator.Validate(IModel model, IDiagnosticsLogger`1 logger)
   at Microsoft.EntityFrameworkCore.Infrastructure.RelationalModelValidator.Validate(IModel model, IDiagnosticsLogger`1 logger)
   at Microsoft.EntityFrameworkCore.SqlServer.Internal.SqlServerModelValidator.Validate(IModel model, IDiagnosticsLogger`1 logger)
   at Microsoft.EntityFrameworkCore.Metadata.Conventions.ValidatingConvention.ProcessModelFinalized(IConventionModelBuilder modelBuilder, IConventionContext`1 context)
   at Microsoft.EntityFrameworkCore.Metadata.Conventions.Internal.ConventionDispatcher.ImmediateConventionScope.OnModelFinalized(IConventionModelBuilder modelBuilder)
   at Microsoft.EntityFrameworkCore.Metadata.Conventions.Internal.ConventionDispatcher.OnModelFinalized(IConventionModelBuilder modelBuilder)
   at Microsoft.EntityFrameworkCore.Metadata.Internal.Model.FinalizeModel()
   at Microsoft.EntityFrameworkCore.ModelBuilder.FinalizeModel()
   at Microsoft.EntityFrameworkCore.Infrastructure.ModelSource.CreateModel(DbContext context, IConventionSetBuilder conventionSetBuilder)
   at Microsoft.EntityFrameworkCore.Infrastructure.ModelSource.GetModel(DbContext context, IConventionSetBuilder conventionSetBuilder)
   at Microsoft.EntityFrameworkCore.Internal.DbContextServices.CreateModel()
   at Microsoft.EntityFrameworkCore.Internal.DbContextServices.get_Model()
   at Microsoft.EntityFrameworkCore.Infrastructure.EntityFrameworkServicesBuilder.<>c.<TryAddCoreServices>b__7_3(IServiceProvider p)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitFactory(FactoryCallSite factoryCallSite, RuntimeResolverContext context)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor`2.VisitCallSiteMain(ServiceCallSite callSite, TArgument argument)
The entity type 'CourseInstructor' requires a primary key to be defined. If you intended to use a keyless entity type call 'HasNoKey()'.
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitCache(ServiceCallSite callSite, RuntimeResolverContext context, ServiceProviderEngineScope serviceProviderEngine, RuntimeResolverLock lockType)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitScopeCache(ServiceCallSite singletonCallSite, RuntimeResolverContext context)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor`2.VisitCallSite(ServiceCallSite callSite, TArgument argument)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitConstructor(ConstructorCallSite constructorCallSite, RuntimeResolverContext context)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor`2.VisitCallSiteMain(ServiceCallSite callSite, TArgument argument)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitCache(ServiceCallSite callSite, RuntimeResolverContext context, ServiceProviderEngineScope serviceProviderEngine, RuntimeResolverLock lockType)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitScopeCache(ServiceCallSite singletonCallSite, RuntimeResolverContext context)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor`2.VisitCallSite(ServiceCallSite callSite, TArgument argument)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.Resolve(ServiceCallSite callSite, ServiceProviderEngineScope scope)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.DynamicServiceProviderEngine.<>c__DisplayClass1_0.<RealizeService>b__0(ServiceProviderEngineScope scope)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.ServiceProviderEngine.GetService(Type serviceType, ServiceProviderEngineScope serviceProviderEngineScope)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.ServiceProviderEngineScope.GetService(Type serviceType)
   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.DbContext.Microsoft.EntityFrameworkCore.Infrastructure.IInfrastructure<System.IServiceProvider>.get_Instance()
   at Microsoft.EntityFrameworkCore.Infrastructure.Internal.InfrastructureExtensions.GetService[TService](IInfrastructure`1 accessor)
   at Microsoft.EntityFrameworkCore.Infrastructure.AccessorExtensions.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.DbContextActivator.CreateInstance(Type contextType, Assembly startupAssembly, IOperationReportHandler reportHandler)
   at Microsoft.VisualStudio.Web.CodeGeneration.EntityFrameworkCore.EntityFrameworkModelProcessor.TryCreateContextUsingAppCode(Type dbContextType, Type startupType)

   at Microsoft.VisualStudio.Web.CodeGeneration.ActionInvoker.<BuildCommandLine>b__6_0()
   at Microsoft.Extensions.CommandLineUtils.CommandLineApplication.Execute(String[] args)
   at Microsoft.VisualStudio.Web.CodeGeneration.ActionInvoker.Execute(String[] args)
   at Microsoft.VisualStudio.Web.CodeGeneration.CodeGenCommand.Execute(String[] args)
RunTime 00:00:05.54

Expected behavior:

I expected to get an error, it because there is no myContext in my project.

Actual behavior:

The log message said below. In fact, there is no myContext been created in the project.

Generating a new DbContext class 'myContext'

Also, there is a ConnectionStrings called myContext has been added to the appsettings.json. What is this for?

{
  "Logging": {
    "LogLevel": {
      "Default": "Information",
      "Microsoft": "Warning",
      "Microsoft.Hosting.Lifetime": "Information"
    }
  },
  "AllowedHosts": "*",
  "ConnectionStrings": {
    "myContext": "Server=(localdb)\\mssqllocaldb;Database=myContext-947196aa-5db2-4bca-8c97-137e835535e7;Trusted_Connection=True;MultipleActiveResultSets=true"
  }
}

Additional information about the project being scaffolded, such as:

Target framework(s): .NET Core 3.1

Package version of Microsoft.AspNetCore.App or Microsoft.AspNetCore.All (if applicable):

Package version of Microsoft.VisualStudio.Web.CodeGeneration.Design - this may be added to your project by scaffolding:

[deepchoudhery]

If the context does not exist, we create one for the model class specified.

[doggy8088]

@deepchoudhery I think the appsettings.json shouldn't be touched if the code generation fail.

[doggy8088]

@deepchoudhery by the way, the default output directory of the created data context class is Data/. Is it possible add an new option to change the default?

[deepchoudhery]

@deepchoudhery I think the appsettings.json shouldn't be touched if the code generation fail.

Yep that makes sense. I'll take a look at that.

[deepchoudhery]

@deepchoudhery by the way, the default output directory of the created data context class is Data/. Is it possible add an new option to change the default?

Does not exist currently, but I could look at this in a servicing update.