Musify/backend/Core/Musify.Application/Users/GetUserFollowers.cs

using ErrorOr;
using Mediator;
using Microsoft.EntityFrameworkCore;
using Musify.Application.Contracts;
using Musify.Application.Shared;
using Musify.Application.Users.Responses;
using X.PagedList.EF;

namespace Musify.Application.Users
{
    public record GetUserFollowersQuery(long? ViewerId, long UserId, int PageNumber, int PageSize)
        : IQuery<ErrorOr<PaginatedResponse<UserSummaryResponse>>>;

    public class GetUserFollowersQueryHandler(IDatabase database)
        : IQueryHandler<GetUserFollowersQuery, ErrorOr<PaginatedResponse<UserSummaryResponse>>>
    {
        public async ValueTask<ErrorOr<PaginatedResponse<UserSummaryResponse>>> Handle(GetUserFollowersQuery request, CancellationToken cancellationToken)
        {
            if (!await database.Users.AnyAsync(u => u.Id == request.UserId, cancellationToken))
                return Error.NotFound();

            if (!await FollowVisibility.CanViewFollowersAsync(database, request.ViewerId, request.UserId, cancellationToken))
                return Error.Forbidden();

            var followsQuery = database.UserFollows
                .AsNoTracking()
                .Where(f => f.FollowedId == request.UserId);

            var totalCount = await followsQuery.CountAsync(cancellationToken);
            var viewerId = request.ViewerId;

            var page = await followsQuery
                .OrderByDescending(f => f.CreatedAt)
                .Select(f => new UserSummaryResponse(
                    f.Follower.Id,
                    f.Follower.Name,
                    f.Follower.FirstName,
                    f.Follower.SecondName,
                    f.Follower.ProfilePictureUrl,
                    viewerId != null && database.UserFollows.Any(v => v.FollowerId == viewerId && v.FollowedId == f.FollowerId)))
                .ToPagedListAsync(request.PageNumber, request.PageSize, totalCount, cancellationToken);

            return PaginatedResponse<UserSummaryResponse>.FromPagedList(page);
        }
    }
}