Budgeteer/BudgetApp/Services/SalaryService.cs

using Microsoft.EntityFrameworkCore;
using BudgetApp.Data;

namespace BudgetApp.Services;

public class SalaryService(ApplicationDbContext db)
{
    private readonly ApplicationDbContext _db = db;

    public async Task<List<Salary>> GetSalariesAsync(string userId)
    {
        return await _db.Salaries
            .Include(s => s.Account)
            .Include(s => s.Category)
            .Where(s => s.UserId == userId)
            .OrderBy(s => s.Name)
            .ToListAsync();
    }

    public async Task CreateSalaryAsync(Salary salary, DateTime windowEnd)
    {
        var salaryCategoryId = await _db.Categories
            .Where(c => c.UserId == salary.UserId && c.Name == "Salary")
            .Select(c => (int?)c.Id)
            .FirstOrDefaultAsync();
        salary.CategoryId = salaryCategoryId;

        _db.Salaries.Add(salary);
        await _db.SaveChangesAsync();
        await MaterializeSalaryTransactionsAsync(salary.UserId, windowEnd);
    }

    public async Task DeleteSalaryAsync(int id, string userId)
    {
        var exists = await _db.Salaries.AnyAsync(s => s.Id == id && s.UserId == userId);
        if (!exists) return;

        await using var transaction = await _db.Database.BeginTransactionAsync();

        // Delete future unpaid transactions generated by this salary
        await _db.Transactions
            .Where(t => t.SalaryId == id && t.Date.Date > DateTime.Today && !t.IsPaid)
            .ExecuteDeleteAsync();

        // Unlink past / paid transactions so they stay in history without the salary reference
        await _db.Transactions
            .Where(t => t.SalaryId == id)
            .ExecuteUpdateAsync(s => s.SetProperty(t => t.SalaryId, (int?)null));

        await _db.Salaries
            .Where(s => s.Id == id && s.UserId == userId)
            .ExecuteDeleteAsync();

        await transaction.CommitAsync();
    }

    public async Task MaterializeSalaryTransactionsAsync(string userId, DateTime windowEnd)
    {
        var salaries = await _db.Salaries
            .Where(s => s.UserId == userId)
            .ToListAsync();

        var newTransactions = new List<Transaction>();

        foreach (var salary in salaries)
        {
            var latestDate = await _db.Transactions
                .Where(t => t.SalaryId == salary.Id)
                .OrderByDescending(t => t.Date)
                .Select(t => (DateTime?)t.Date)
                .FirstOrDefaultAsync();

            var nextDate = latestDate.HasValue
                ? GetNextDate(latestDate.Value, salary)
                : salary.StartDate.Date;

            while (nextDate <= windowEnd)
            {
                newTransactions.Add(new Transaction
                {
                    Description = salary.Name,
                    Amount = salary.Amount,
                    Date = nextDate,
                    Type = TransactionType.Income,
                    AccountId = salary.AccountId,
                    CategoryId = salary.CategoryId,
                    RecurrenceType = RecurrenceType.None,
                    SalaryId = salary.Id
                });
                nextDate = GetNextDate(nextDate, salary);
            }
        }

        if (newTransactions.Count > 0)
        {
            _db.Transactions.AddRange(newTransactions);
            await _db.SaveChangesAsync();
        }
    }

    public async Task<DateTime?> GetNextPaydateAsync(int salaryId)
    {
        return await _db.Transactions
            .Where(t => t.SalaryId == salaryId && t.Date.Date >= DateTime.Today)
            .OrderBy(t => t.Date)
            .Select(t => (DateTime?)t.Date)
            .FirstOrDefaultAsync();
    }

    private static DateTime GetNextDate(DateTime from, Salary salary) => salary.RecurrenceType switch
    {
        RecurrenceType.Weekly  => from.AddDays(7),
        RecurrenceType.Monthly => from.AddMonths(1),
        RecurrenceType.Yearly  => from.AddYears(1),
        _                      => from.AddMonths(1)
    };
}