ANTOOZA-BACKEND/Application/Features/Api/Member/Follow/Toggle/Handler.cs

using Application.Abstractions.Data;
using Application.Abstractions.Messaging;
using Microsoft.EntityFrameworkCore;
using SharedKernel.Results;

namespace Application.Features.Api.Member.Follow.Toggle;

public sealed class Handler(IAppDbContext db) : ICommandHandler<Command, Result<Response>>
{
    public async Task<Result<Response>> Handle(Command request, CancellationToken ct)
    {
        var followee = await db.Member.AsNoTracking().Where(x => x.SID == request.FolloweeSID && !x.IsWithdraw).Select(x => new { x.ID }).FirstOrDefaultAsync(ct);

        if (followee is null)
        {
            return Result.Failure<Response>(Error.NotFound("Follow.MemberNotFound", "회원을 찾을 수 없습니다."));
        }

        if (followee.ID == request.FollowerMemberID)
        {
            return Result.Failure<Response>(Error.Problem("Follow.Self", "자기 자신을 팔로우할 수 없습니다."));
        }

        var existing = await db.MemberFollow.FirstOrDefaultAsync(x => x.FollowerMemberID == request.FollowerMemberID && x.FolloweeMemberID == followee.ID, ct);

        var followerStats = await db.MemberStats.FirstOrDefaultAsync(x => x.MemberID == request.FollowerMemberID, ct);
        var followeeStats = await db.MemberStats.FirstOrDefaultAsync(x => x.MemberID == followee.ID, ct);

        // 기존 회원이 MemberStats 없는 경우 자가 복구 — 과거 가입 경로에서 누락된 데이터 메우기
        if (followerStats is null)
        {
            followerStats = Domain.Entities.Members.MemberStats.Create(request.FollowerMemberID);
            await db.MemberStats.AddAsync(followerStats, ct);
        }

        if (followeeStats is null)
        {
            followeeStats = Domain.Entities.Members.MemberStats.Create(followee.ID);
            await db.MemberStats.AddAsync(followeeStats, ct);
        }

        bool isFollowing;

        if (existing is not null)
        {
            db.MemberFollow.Remove(existing);
            followerStats.FollowingCount = Math.Max(0, followerStats.FollowingCount - 1);
            followeeStats.FollowerCount = Math.Max(0, followeeStats.FollowerCount - 1);
            isFollowing = false;
        }
        else
        {
            var follow = Domain.Entities.Members.MemberFollow.Create(request.FollowerMemberID, followee.ID);
            db.MemberFollow.Add(follow);
            followerStats.FollowingCount += 1;
            followeeStats.FollowerCount += 1;
            isFollowing = true;
        }

        await db.SaveChangesAsync(ct);

        return new Response(isFollowing, followeeStats.FollowerCount);
    }
}