Web Application Development Tutorial - Part 6: Authors: Domain Layer

Introduction

In the previous parts, we've used the ABP infrastructure to easily build some services;

• Used the CrudAppService base class instead of manually developing an application service for standard create, read, update and delete operations.
• Used generic repositories to completely automate the database layer.

For the "Authors" part;

• We will do some of the things manually to show how you can do it in case of need.
• We will implement some Domain Driven Design (DDD) best practices.

The development will be done layer by layer to concentrate on an individual layer in one time. In a real project, you will develop your application feature by feature (vertical) as done in the previous parts. In this way, you will experience both approaches.

The Author Entity

Create an Authors folder (namespace) in the Acme.BookStore.Domain project and add an Author class inside it:

using System;
using JetBrains.Annotations;
using Volo.Abp;
using Volo.Abp.Domain.Entities.Auditing;

namespace Acme.BookStore.Authors;

public class Author : FullAuditedAggregateRoot<Guid>
{
    public string Name { get; private set; }
    public DateTime BirthDate { get; set; }
    public string ShortBio { get; set; }

    private Author()
    {
        /* This constructor is for deserialization / ORM purpose */
    }

    internal Author(
        Guid id,
        string name,
        DateTime birthDate,
        string? shortBio = null)
        : base(id)
    {
        SetName(name);
        BirthDate = birthDate;
        ShortBio = shortBio;
    }

    internal Author ChangeName(string name)
    {
        SetName(name);
        return this;
    }

    private void SetName(string name)
    {
        Name = Check.NotNullOrWhiteSpace(
            name, 
            nameof(name), 
            maxLength: AuthorConsts.MaxNameLength
        );
    }
}

• Inherited from FullAuditedAggregateRoot<Guid> which makes the entity soft delete (that means when you delete it, it is not deleted in the database, but just marked as deleted) with all the auditing properties.
• private set for the Name property restricts to set this property from out of this class. There are two ways of setting the name (in both cases, we validate the name):
  • In the constructor, while creating a new author.
  • Using the ChangeName method to update the name later.
• The constructor and the ChangeName method is internal to force to use these methods only in the domain layer, using the AuthorManager that will be explained later.
• Check class is an ABP utility class to help you while checking method arguments (it throws ArgumentException on an invalid case).

AuthorConsts is a simple class that is located under the Authors namespace (folder) of the Acme.BookStore.Domain.Shared project:

namespace Acme.BookStore.Authors;

public static class AuthorConsts
{
    public const int MaxNameLength = 64;
}

Created this class inside the Acme.BookStore.Domain.Shared project since we will re-use it on the Data Transfer Objects (DTOs) later.

AuthorManager: The Domain Service

Author constructor and ChangeName methods are internal, so they can be used only in the domain layer. Create an AuthorManager class in the Authors folder (namespace) of the Acme.BookStore.Domain project:

using System;
using System.Threading.Tasks;
using JetBrains.Annotations;
using Volo.Abp;
using Volo.Abp.Domain.Services;

namespace Acme.BookStore.Authors;

public class AuthorManager : DomainService
{
    private readonly IAuthorRepository _authorRepository;

    public AuthorManager(IAuthorRepository authorRepository)
    {
        _authorRepository = authorRepository;
    }

    public async Task<Author> CreateAsync(
        string name,
        DateTime birthDate,
        string? shortBio = null)
    {
        Check.NotNullOrWhiteSpace(name, nameof(name));

        var existingAuthor = await _authorRepository.FindByNameAsync(name);
        if (existingAuthor != null)
        {
            throw new AuthorAlreadyExistsException(name);
        }

        return new Author(
            GuidGenerator.Create(),
            name,
            birthDate,
            shortBio
        );
    }

    public async Task ChangeNameAsync(
        Author author,
        string newName)
    {
        Check.NotNull(author, nameof(author));
        Check.NotNullOrWhiteSpace(newName, nameof(newName));

        var existingAuthor = await _authorRepository.FindByNameAsync(newName);
        if (existingAuthor != null && existingAuthor.Id != author.Id)
        {
            throw new AuthorAlreadyExistsException(newName);
        }

        author.ChangeName(newName);
    }
}

• AuthorManager forces to create an author and change name of an author in a controlled way. The application layer (will be introduced later) will use these methods.

DDD tip: Do not introduce domain service methods unless they are really needed and perform some core business rules. For this case, we needed this service to be able to force the unique name constraint.

Both methods checks if there is already an author with the given name and throws a special business exception, AuthorAlreadyExistsException, defined in the Acme.BookStore.Domain project (in the Authors folder) as shown below:

using Volo.Abp;

namespace Acme.BookStore.Authors;

public class AuthorAlreadyExistsException : BusinessException
{
    public AuthorAlreadyExistsException(string name)
        : base(BookStoreDomainErrorCodes.AuthorAlreadyExists)
    {
        WithData("name", name);
    }
}

BusinessException is a special exception type. It is a good practice to throw domain related exceptions when needed. It is automatically handled by the ABP and can be easily localized. WithData(...) method is used to provide additional data to the exception object that will later be used on the localization message or for some other purpose.

Open the BookStoreDomainErrorCodes in the Acme.BookStore.Domain.Shared project and change as shown below:

namespace Acme.BookStore;

public static class BookStoreDomainErrorCodes
{
    public const string AuthorAlreadyExists = "BookStore:00001";
}

This is a unique string represents the error code thrown by your application and can be handled by client applications. For users, you probably want to localize it. Open the Localization/BookStore/en.json inside the Acme.BookStore.Domain.Shared project and add the following entry:

"BookStore:00001": "There is already an author with the same name: {name}"

Whenever you throw an AuthorAlreadyExistsException, the end user will see a nice error message on the UI.

IAuthorRepository

AuthorManager injects the IAuthorRepository, so we need to define it. Create this new interface in the Authors folder (namespace) of the Acme.BookStore.Domain project:

using System;
using System.Collections.Generic;
using System.Threading.Tasks;
using Volo.Abp.Domain.Repositories;

namespace Acme.BookStore.Authors;

public interface IAuthorRepository : IRepository<Author, Guid>
{
    Task<Author> FindByNameAsync(string name);

    Task<List<Author>> GetListAsync(
        int skipCount,
        int maxResultCount,
        string sorting,
        string filter = null
    );
}

• IAuthorRepository extends the standard IRepository<Author, Guid> interface, so all the standard repository methods will also be available for the IAuthorRepository.
• FindByNameAsync was used in the AuthorManager to query an author by name.
• GetListAsync will be used in the application layer to get a listed, sorted and filtered list of authors to show on the UI.

We will implement this repository in the next part.

Both of these methods might seem unnecessary since the standard repositories already provide generic querying methods and you can easily use them instead of defining such custom methods. You're right and do it like in a real application. However, for this "learning" tutorial, it is useful to explain how to create custom repository methods when you really need it.

Conclusion

This part covered the domain layer of the authors functionality of the book store application. The main files created/updated in this part was highlighted in the picture below: