classlib/ExpenseTracker.Application/Accounts/Queries/Export/Csv/One/ExportCsvAccountQueryHandler.cs

using System.Globalization;
using System.Text;
using AutoMapper;
using CsvHelper;
using MediatR;
using ExpenseTracker.Application.Common.Exceptions;
using ExpenseTracker.Application.Common.Interfaces.Repositories;
using ExpenseTracker.Domain.Entities;

namespace ExpenseTracker.Application.Accounts.Queries.Export.Csv.One;

public class ExportCsvAccountQueryHandler : IRequestHandler<ExportCsvAccountQuery, byte[]>
{
    private readonly IMapper _mapper;
    private readonly IAccountRepository _accountRepository;
    private readonly ITransactionRepository _transactionRepository;

    public ExportCsvAccountQueryHandler(
        IMapper mapper,
        IAccountRepository repository,
        ITransactionRepository transactionRepository)
    {
        _mapper = mapper;
        _accountRepository = repository;
        _transactionRepository = transactionRepository;
    }

    public async Task<byte[]> Handle(ExportCsvAccountQuery request, CancellationToken cancellationToken)
    {
        var entity = _accountRepository.Queryable
            .GroupJoin(
                _transactionRepository.Queryable,
                b => b.Id,
                t => t.AccountId,
                (account, transactions) =>
                    new Account
                    {
                        Id = account.Id,
                        Name = account.Name,
                        Description = account.Description,
                        UserId = account.UserId,
                        Transactions = transactions
                    }
            )
            .FirstOrDefault(e => e.Id == request.Id);

        // TODO: Mapping to DTO creates new objects therefore using more resources
        // Create custom csv serializer to avoid mapping

        var entityDto = _mapper.Map<AccountJsonDto>(entity);

        using (var memoryStream = new MemoryStream())
        {
            using (var streamWriter = new StreamWriter(memoryStream, Encoding.UTF8))
            {
                using (var csvWriter = new CsvWriter(streamWriter, CultureInfo.InvariantCulture))
                {
                    csvWriter.Context.RegisterClassMap<TransactionCsvMap>();
                    await csvWriter.WriteRecordsAsync(entityDto.Transactions);
                    await csvWriter.FlushAsync();
                }
            }

            return memoryStream.ToArray();
        }
    }
}