Entity Framework Core Scaffolding with Handlebars

Scaffold EF Core models using Handlebars templates.

• Uses Handlebars.NET to compile Handlebars templates when generating models with the Entity Framework Core scaffolding tools.

Prerequisites

• Visual Studio 2017 15.9 or greater.
• The .NET Core 2.2 SDK (version 2.2.100 or greater).

Database Setup

1. Use SQL Server Management Studio to connect to SQL Server
   • The easiest is to use LocalDb, which is installed with Visual Studio.
     Connect to: (localdb)\MsSqlLocalDb.
   • Create a new database named NorthwindSlim.
   • Download the data file from http://bit.ly/northwindslim.
   • Unzip NorthwindSlim.sql and run the script to create tables and populate them with data.

Usage

1. Create a new .NET Core class library.

   • If necessary, edit the csproj file to update the TargetFramework to 2.2.

   Note: Using the EF Core toolchain with a .NET Standard class library is currently not supported. Instead, you can add a .NET Standard class library to the same solution as the .NET Core library, then add existing items and select Add As Link to include entity classes.

2. Add EF Core SQL Server and Tools NuGet packages.

   • Open the Package Manager Console, select the default project and enter:
     • Install-Package Microsoft.EntityFrameworkCore.SqlServer
     • Install-Package Microsoft.EntityFrameworkCore.Design
   • If needed update EF Core to version 2.2.

3. Add the EntityFrameworkCore.Scaffolding.Handlebars NuGet package:

   • Install-Package EntityFrameworkCore.Scaffolding.Handlebars

4. Remove Class1.cs and add a ScaffoldingDesignTimeServices class.

   • Implement IDesignTimeServices by adding a ConfigureDesignTimeServices method that calls services.AddHandlebarsScaffolding.
   • You can optionally pass a ReverseEngineerOptions enum to indicate if you wish to generate only entity types, only a DbContext class, or both (which is the default).

   public class ScaffoldingDesignTimeServices : IDesignTimeServices
   {
       public void ConfigureDesignTimeServices(IServiceCollection services)
       {
           var options = ReverseEngineerOptions.DbContextAndEntities;
           services.AddHandlebarsScaffolding(options);
       }
   }

5. Open a command prompt at the project level and use the EF .NET Core CLI tools to reverse engineer a context and models from an existing database.

   • Get help on dotnet-ef-dbcontext-scaffold at the command line: dotnet ef dbcontext scaffold -h
   • Execute the following command to reverse engineer classes from the NorthwindSlim database:

   dotnet ef dbcontext scaffold "Data Source=(localdb)\MSSQLLocalDB; Initial Catalog=NorthwindSlim; Integrated Security=True" Microsoft.EntityFrameworkCore.SqlServer -o Models -c NorthwindSlimContext -f --context-dir Contexts
   

   • You should see context and/or entity classes appear in the Models folder of the project.
   • You will also see a CodeTemplates folder appear containing Handlebars templates for customizing generation of context and entity type classes.
   • Add -d to the command to use data annotations. You will need to add the System.ComponentModel.Annotations package to a .NET Standard library containing linked entity classes.

6. You may edit any of the template files which appear under the CodeTemplates folder.

   • For now you can just add some comments, but you may wish to customize the templates in other ways, for example, by inheriting entities from a base class or implementing specific interfaces.
   • When you run the dotnet-ef-dbcontext-scaffold command again, you will see your updated reflected in the generated classes.

Handlebars Helpers and Transformers

You can register Handlebars helpers in the ScaffoldingDesignTimeServices where setup takes place.

• Create a named tuple as shown with myHelper below.
• The context parameter of the helper method provides model data injected by the Handlebars scaffolding extension.
• Pass the tuple to the AddHandlebarsHelpers extension method.
• To use Handlebars helper defined above, add the following to any of the .hbs files within the CodeTemplates folder: {{my-helper}}
• You may register as many helpers as you wish.

You can pass transform functions to AddHandlebarsTransformers in order to customize generation of entity type definitions, including class names, constructors and properties.

public class ScaffoldingDesignTimeServices : IDesignTimeServices
{
    public void ConfigureDesignTimeServices(IServiceCollection services)
    {
        // Generate both context and entities
        var options = ReverseEngineerOptions.DbContextAndEntities;

        // Register Handlebars helper
        var myHelper = (helperName: "my-helper", helperFunction: (Action<TextWriter, Dictionary<string, object>, object[]>) MyHbsHelper);

        // Add Handlebars scaffolding templates
        services.AddHandlebarsScaffolding(options);

        // Add optional Handlebars helpers
        services.AddHandlebarsHelpers(myHelper);

        // Add Handlebars transformer for Country property
        services.AddHandlebarsTransformers(
            propertyTransformer: e =>
                e.PropertyName == "Country"
                    ? new EntityPropertyInfo("Country", e.PropertyName)
                    : new EntityPropertyInfo(e.PropertyType, e.PropertyName));

        // Add optional Handlebars transformers
        //services.AddHandlebarsTransformers(
        //    entityNameTransformer: n => n + "Foo",
        //    entityFileNameTransformer: n => n + "Foo",
        //    constructorTransformer: e => new EntityPropertyInfo(e.PropertyType + "Foo", e.PropertyName + "Foo"),
        //    propertyTransformer: e => new EntityPropertyInfo(e.PropertyType, e.PropertyName + "Foo"),
        //    navPropertyTransformer: e => new EntityPropertyInfo(e.PropertyType + "Foo", e.PropertyName + "Foo"));
    }

    // Sample Handlebars helper
    void MyHbsHelper(TextWriter writer, Dictionary<string, object> context, object[] parameters)
    {
        writer.Write("// My Handlebars Helper");
    }
}

Extending the OnModelCreating Method

There are times when you might like to modify generated code, for example, by adding a HasConversion method to an entity property in the OnModelCreating method of the generated class that extends DbContext. However, doing so may prove futile because added code would be overwritten the next time you run the dotnet ef dbcontext scaffold command.

• Rather than modifying generated code, a better idea would be to extend it by using partial classes and methods. To enable this scenario, the generated DbContext class is already defined using the partial keyword, and it contains a partial OnModelCreatingExt method that is invoked at the end of the OnModelCreating method.
• To implement the partial method, simply add a new class to your project with the same name as the generated DbContext class, and define it as partial. Then add a OnModelCreatingExt method with the same signature as the partial method defined in the generated DbContext class.

// Place in separate class file (NorthwindSlimContextExt.cs)
public partial class NorthwindSlimContext
{
    partial void OnModelCreatingExt(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Employee>()
            .Property(e => e.Country)
            .HasConversion(
                v => v.ToString(),
                v => (Country)Enum.Parse(typeof(Country), v));

        modelBuilder.Entity<Customer>()
            .Property(e => e.Country)
            .HasConversion(
                v => v.ToString(),
                v => (Country)Enum.Parse(typeof(Country), v));
    }
}