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

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

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

public sealed class Handler(IAppDbContext db) : IQueryHandler<Query, Result<Response>>
{
    public async Task<Result<Response>> Handle(Query request, CancellationToken ct)
    {
        if (request.FolloweeSIDs.Count == 0)
        {
            return Result.Success(new Response { Map = new Dictionary<string, bool>() });
        }

        var sids = request.FolloweeSIDs.Distinct().ToArray();

        var targets = await db.Member.AsNoTracking().Where(x => sids.Contains(x.SID)).Select(x => new { x.ID, x.SID }).ToListAsync(ct);

        if (targets.Count == 0)
        {
            return Result.Success(new Response { Map = sids.ToDictionary(s => s, _ => false) });
        }

        var targetIDs = targets.Select(x => x.ID).ToArray();

        var followingIDs = await db.MemberFollow.AsNoTracking()
            .Where(x => x.FollowerMemberID == request.FollowerMemberID && targetIDs.Contains(x.FolloweeMemberID))
            .Select(x => x.FolloweeMemberID)
            .ToListAsync(ct);

        var followingSet = followingIDs.ToHashSet();
        var sidToId = targets.ToDictionary(x => x.SID, x => x.ID);

        var map = sids.ToDictionary(
            sid => sid,
            sid => sidToId.TryGetValue(sid, out var id) && followingSet.Contains(id)
        );

        return Result.Success(new Response { Map = map });
    }
}