Repositories

"Mediates between the domain and data mapping layers using a collection-like interface for accessing domain objects" (Martin Fowler).

Repositories, in practice, are used to perform database operations for domain objects (see Entities). Generally, a separated repository is used for each aggregate root or entity.

Generic Repositories

ABP can provide a default generic repository for each aggregate root or entity. You can inject IRepository<TEntity, TKey> into your service and perform standard CRUD operations.

Database provider layer should be properly configured to be able to use the default generic repositories. It is already done if you've created your project using the startup templates. If not, refer to the database provider documents (EF Core / MongoDB) to configure it.

Example usage of a default generic repository:

public class PersonAppService : ApplicationService
{
    private readonly IRepository<Person, Guid> _personRepository;

    public PersonAppService(IRepository<Person, Guid> personRepository)
    {
        _personRepository = personRepository;
    }

    public async Task Create(CreatePersonDto input)
    {
        var person = new Person { Name = input.Name, Age = input.Age };

        await _personRepository.InsertAsync(person);
    }

    public List<PersonDto> GetList(string nameFilter)
    {
        var people = _personRepository
            .Where(p => p.Name.Contains(nameFilter))
            .ToList();

        return people
            .Select(p => new PersonDto {Id = p.Id, Name = p.Name, Age = p.Age})
            .ToList();
    }
}

See the "IQueryable & Async Operations" section below to understand how you can use async extension methods, like ToListAsync() (which is strongly suggested) instead of ToList().

In this example;

• PersonAppService simply injects IRepository<Person, Guid> in it's constructor.
• Create method uses InsertAsync to save a newly created entity.
• GetList method uses the standard LINQ Where and ToList methods to filter and get a list of people from the data source.

The example above uses hand-made mapping between entities and DTOs. See object to object mapping document for an automatic way of mapping.

Generic Repositories provides some standard CRUD features out of the box:

• Provides Insert method to save a new entity.
• Provides Update and Delete methods to update or delete an entity by entity object or it's id.
• Provides Delete method to delete multiple entities by a filter.
• Implements IQueryable<TEntity>, so you can use LINQ and extension methods like FirstOrDefault, Where, OrderBy, ToList and so on...

Basic Repositories

Standard IRepository<TEntity, TKey> interface extends standard IQueryable<TEntity> and you can freely query using standard LINQ methods. However, some ORM providers or database systems may not support standard IQueryable interface.

ABP provides IBasicRepository<TEntity, TPrimaryKey> and IBasicRepository<TEntity> interfaces to support such scenarios. You can extend these interfaces (and optionally derive from BasicRepositoryBase) to create custom repositories for your entities.

Depending on IBasicRepository but not depending on IRepository has an advantage to make possible to work with all data sources even if they don't support IQueryable. But major vendors, like Entity Framework, NHibernate or MongoDb already support IQueryable.

So, working with IRepository is the suggested way for typical applications. But reusable module developers may consider IBasicRepository to support a wider range of data sources.

Read Only Repositories

There are also IReadOnlyRepository<TEntity, TKey> and IReadOnlyBasicRepository<Tentity, TKey> interfaces for who only want to depend on querying capabilities of the repositories.

Generic Repository without a Primary Key

If your entity does not have an Id primary key (it may have a composite primary key for instance) then you cannot use the IRepository<TEntity, TKey> (or basic/readonly versions) defined above. In that case, you can inject and use IRepository<TEntity> for your entity.

IRepository<TEntity> has a few missing methods those normally works with the Id property of an entity. Because of the entity has no Id property in that case, these methods are not available. One example is the Get method that gets an id and returns the entity with given id. However, you can still use IQueryable<TEntity> features to query entities by standard LINQ methods.

Custom Repositories

Default generic repositories will be sufficient for most cases. However, you may need to create a custom repository class for your entity.

Custom Repository Example

ABP does not force you to implement any interface or inherit from any base class for a repository. It can be just a simple POCO class. However, it's suggested to inherit existing repository interface and classes to make your work easier and get the standard methods out of the box.

Custom Repository Interface

First, define an interface in your domain layer:

public interface IPersonRepository : IRepository<Person, Guid>
{
    Task<Person> FindByNameAsync(string name);
}

This interface extends IRepository<Person, Guid> to take advantage of pre-built repository functionality.

Custom Repository Implementation

A custom repository is tightly coupled to the data access tool type you are using. In this example, we will use Entity Framework Core:

public class PersonRepository : EfCoreRepository<MyDbContext, Person, Guid>, IPersonRepository
{
    public PersonRepository(IDbContextProvider<TestAppDbContext> dbContextProvider) 
        : base(dbContextProvider)
    {

    }

    public async Task<Person> FindByNameAsync(string name)
    {
        return await DbContext.Set<Person>()
            .Where(p => p.Name == name)
            .FirstOrDefaultAsync();
    }
}

You can directly access the data access provider (DbContext in this case) to perform operations.

See EF Core or MongoDb document for more info about the custom repositories.

IQueryable & Async Operations

IRepository inherits from IQueryable, that means you can directly use LINQ extension methods on it, as shown in the example of the "Generic Repositories" section above.

Example: Using the Where(...) and the ToList() extension methods

var people = _personRepository
    .Where(p => p.Name.Contains(nameFilter))
    .ToList();

.ToList, Count()... are standard extension methods defined in the System.Linq namespace (see all).

You normally want to use .ToListAsync(), .CountAsync()... instead, to be able to write a truly async code.

However, you see that you can't use all the async extension methods in your application or domain layer when you create a new project using the standard application startup template, because;

• These async methods are not standard LINQ methods and they are defined in the Microsoft.EntityFrameworkCore NuGet package.
• The standard template doesn't have a reference to the EF Core package from the domain and application layers, to be independent from the database provider.

Based on your requirements and development model, you have the following options to be able to use the async methods.

Using async methods is strongly suggested! Don't use sync LINQ methods while executing database queries to be able to develop a scalable application.

Option-1: Reference to the Database Provider Package

The easiest solution is to directly add the EF Core package from the project you want to use these async methods.

Add the Volo.Abp.EntityFrameworkCore NuGet package to your project, which indirectly reference to the EF Core package. This ensures that you use the correct version of the EF Core compatible to the rest of your application.

When you add the NuGet package to your project, you can take full power of the EF Core extension methods.

Example: Directly using the ToListAsync() after adding the EF Core package

var people = _personRepository
    .Where(p => p.Name.Contains(nameFilter))
    .ToListAsync();

This method is suggested;

• If you are developing an application and you don't plan to change EF Core in the future, or you can tolerate it if you need to change later. We believe that's reasonable if you are developing a final application.

MongoDB Case

If you are using MongoDB, you need to add the Volo.Abp.MongoDB NuGet package to your project. Even in this case, you can't directly use async LINQ extensions (like ToListAsync) because MongoDB doesn't provide async extension methods for IQueryable<T>, but provides for IMongoQueryable<T>. You need to cast the query to IMongoQueryable<T> first to be able to use the async extension methods.

Example: Cast IQueryable<T> to IMongoQueryable<T> and use ToListAsync()

var people = ((IMongoQueryable<Person>)_personRepository
    .Where(p => p.Name.Contains(nameFilter)))
    .ToListAsync();

Option-2: Use the IRepository Async Extension Methods

ABP Framework provides async extension methods for the repositories, just similar to async LINQ extension methods.

Example: Use CountAsync and FirstOrDefaultAsync methods on the repositories

var countAll = await _personRepository
    .CountAsync();

var count = await _personRepository
    .CountAsync(x => x.Name.StartsWith("A"));

var book1984 = await _bookRepository
    .FirstOrDefaultAsync(x => x.Name == "John");    

The standard LINQ extension methods are supported: AllAsync, AnyAsync, AverageAsync, ContainsAsync, CountAsync, FirstAsync, FirstOrDefaultAsync, LastAsync, LastOrDefaultAsync, LongCountAsync, MaxAsync, MinAsync, SingleAsync, SingleOrDefaultAsync, SumAsync, ToArrayAsync, ToListAsync.

This approach still has a limitation. You need to call the extension method directly on the repository object. For example, the below usage is not supported:

var count = await _bookRepository.Where(x => x.Name.Contains("A")).CountAsync();

This is because the object returned from the Where method is not a repository object, it is a standard IQueryable interface. See the other options for such cases.

This method is suggested wherever possible.

Option-3: IAsyncQueryableExecuter

IAsyncQueryableExecuter is a service that is used to execute an IQueryable<T> object asynchronously without depending on the actual database provider.

Example: Inject & use the IAsyncQueryableExecuter.ToListAsync() method

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;
using Volo.Abp.Application.Dtos;
using Volo.Abp.Application.Services;
using Volo.Abp.Domain.Repositories;
using Volo.Abp.Linq;

namespace AbpDemo
{
    public class ProductAppService : ApplicationService, IProductAppService
    {
        private readonly IRepository<Product, Guid> _productRepository;
        private readonly IAsyncQueryableExecuter _asyncExecuter;

        public ProductAppService(
            IRepository<Product, Guid> productRepository,
            IAsyncQueryableExecuter asyncExecuter)
        {
            _productRepository = productRepository;
            _asyncExecuter = asyncExecuter;
        }

        public async Task<ListResultDto<ProductDto>> GetListAsync(string name)
        {
            //Create the query
            var query = _productRepository
                .Where(p => p.Name.Contains(name))
                .OrderBy(p => p.Name);

            //Run the query asynchronously
            List<Product> products = await _asyncExecuter.ToListAsync(query);

            //...
        }
    }
}

ApplicationService and DomainService base classes already have AsyncExecuter properties pre-injected and usable without needing an explicit constructor injection.

ABP Framework executes the query asynchronously using the actual database provider's API. While that is not a usual way to execute a query, it is the best way to use the async API without depending on the database provider.

This method is suggested;

• If you want to develop your application code without depending on the database provider.
• If you are building a reusable library that doesn't have a database provider integration package, but needs to execute an IQueryable<T> object in some case.

For example, ABP Framework uses the IAsyncQueryableExecuter in the CrudAppService base class (see the application services document).

Option-4: Custom Repository Methods

You can always create custom repository methods and use the database provider specific APIs, like async extension methods here. See EF Core or MongoDb document for more info about the custom repositories.

This method is suggested;

• If you want to completely isolate your domain & application layers from the database provider.
• If you develop a reusable application module and don't want to force to a specific database provider, which should be done as a best practice.