feat: add a day command

Application/Cqrs/Day/Commands/CreateDay/CreateDayCommand.cs

@@ -0,0 +1,12 @@
+using MediatR;
+using System;
+
+namespace Mirea.Api.DataAccess.Application.Cqrs.Day.Commands.CreateDay;
+
+public class CreateDayCommand : IRequest<int>
+{
+    public required DayOfWeek Index { get; set; }
+    public required int PairNumber { get; set; }
+    public required int LessonId { get; set; }
+    public required int GroupId { get; set; }
+}

Application/Cqrs/Day/Commands/CreateDay/CreateDayCommandHandler.cs

@@ -0,0 +1,38 @@
+using MediatR;
+using Microsoft.EntityFrameworkCore;
+using Mirea.Api.DataAccess.Application.Common.Exceptions;
+using Mirea.Api.DataAccess.Application.Interfaces.DbContexts.Schedule;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace Mirea.Api.DataAccess.Application.Cqrs.Day.Commands.CreateDay;
+
+public class CreateDayCommandHandler(IDayDbContext dbContext) : IRequestHandler<CreateDayCommand, int>
+{
+    public async Task<int> Handle(CreateDayCommand request, CancellationToken cancellationToken)
+    {
+        var entity = await dbContext.Days
+            .FirstOrDefaultAsync(d =>
+                d.LessonId == request.LessonId
+                && d.PairNumber == request.PairNumber
+                && d.GroupId == request.GroupId
+                && d.Index == request.Index,
+                cancellationToken: cancellationToken);
+
+        if (entity != null)
+            throw new RecordExistException(typeof(Domain.Schedule.Day), entity.Id);
+
+        var day = new Domain.Schedule.Day()
+        {
+            Index = request.Index,
+            PairNumber = request.PairNumber,
+            GroupId = request.GroupId,
+            LessonId = request.LessonId
+        };
+
+        var result = await dbContext.Days.AddAsync(day, cancellationToken);
+        await dbContext.SaveChangesAsync(cancellationToken);
+
+        return result.Entity.Id;
+    }
+}