ANTOOZA-BACKEND/Application/Features/Api/Store/Inventory/Groups/Handler.cs

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

namespace Application.Features.Api.Store.Inventory.Groups;

internal sealed class Handler(IAppDbContext db) : IQueryHandler<Query, Response>
{
    public async Task<Response> Handle(Query request, CancellationToken ct)
    {
        // 1단계: ProductID 단위 그룹 + aggregations (EF Core 가 SQL 로 깔끔히 번역).
        //        anonymous key + 다중 join + Response 생성자 projection 을 한 번에 시도하면
        //        translation 실패하므로, 그룹 aggregations 만 먼저 가져온다.
        var groupedQuery = db.MemberInventory.AsNoTracking()
            .Where(i => i.MemberID == request.MemberID && i.DeletedAt == null)
            .GroupBy(i => i.OrderItem!.ProductID)
            .Select(g => new
            {
                ProductID = g.Key,
                TotalCount = g.Count(),
                UnusedCount = g.Count(x => x.UsedAt == null),
                EarliestAcquiredAt = g.Min(x => x.AcquiredAt)
            });

        var total = await groupedQuery.CountAsync(ct);

        var pagedGroups = await groupedQuery
            .OrderByDescending(g => g.EarliestAcquiredAt)
            .Skip((request.Page - 1) * request.PerPage)
            .Take(request.PerPage)
            .ToListAsync(ct);

        if (pagedGroups.Count == 0)
        {
            return new Response(total, []);
        }

        // 2단계: 표시 페이지의 Product + Game 메타데이터 한 번에 fetch.
        var productIDs = pagedGroups.Select(g => g.ProductID).ToList();

        var products = await db.Product.AsNoTracking()
            .Where(p => productIDs.Contains(p.ID))
            .Select(p => new
            {
                p.ID,
                p.Name,
                p.Thumbnail,
                GameName = p.Game!.KorName
            })
            .ToListAsync(ct);

        var productMap = products.ToDictionary(p => p.ID);

        // 3단계: 정렬 순서 유지하면서 stitch.
        var list = pagedGroups.Select(g =>
        {
            var meta = productMap.GetValueOrDefault(g.ProductID);

            return new Response.GroupRow(
                g.ProductID,
                meta?.Name ?? string.Empty,
                meta?.Thumbnail,
                meta?.GameName ?? string.Empty,
                g.TotalCount,
                g.UnusedCount,
                g.EarliestAcquiredAt
            );
        }).ToList();

        return new Response(total, list);
    }
}