step3/api/QuestionsApi/Services/QuestionStore.cs

using MongoDB.Driver;
using QuestionsApi.Models;

namespace QuestionsApi.Services;

public class QuestionStore
{
    private readonly IMongoCollection<Question> _col;

    public QuestionStore(IConfiguration cfg)
    {
        var cs = cfg["MongoDB:ConnectionString"] ?? "mongodb://127.0.0.1:27017";
        var dbName = cfg["MongoDB:DatabaseName"] ?? "GameDb";
        var client = new MongoClient(cs);
        var db = client.GetDatabase(dbName);
        _col = db.GetCollection<Question>("Questions");

        var externalIdIndex = Builders<Question>.IndexKeys.Ascending(q => q.ExternalId);
        _col.Indexes.CreateOne(new CreateIndexModel<Question>(externalIdIndex, new CreateIndexOptions { Unique = true }));
        var systemIndex = Builders<Question>.IndexKeys.Ascending(q => q.System).Ascending(q => q.IsPublished);
        _col.Indexes.CreateOne(new CreateIndexModel<Question>(systemIndex));
        var tagsIndex = Builders<Question>.IndexKeys.Ascending(q => q.Tags);
        _col.Indexes.CreateOne(new CreateIndexModel<Question>(tagsIndex));
    }

    public async Task<List<Question>> SearchAsync(QuestionSearchRequest req, bool includeUnpublished)
    {
        var builder = Builders<Question>.Filter;
        var filters = new List<FilterDefinition<Question>>();

        if (!includeUnpublished)
            filters.Add(builder.Eq(q => q.IsPublished, true));
        if (!string.IsNullOrWhiteSpace(req.System) && req.System != "All Systems")
            filters.Add(builder.Eq(q => q.System, req.System.Trim()));
        if (!string.IsNullOrWhiteSpace(req.Discipline))
            filters.Add(builder.Eq(q => q.Discipline, req.Discipline.Trim()));
        if (req.Tags.Count > 0)
            filters.Add(builder.All(q => q.Tags, req.Tags.Where(t => !string.IsNullOrWhiteSpace(t)).Select(t => t.Trim())));
        if (req.DifficultyMin is not null)
            filters.Add(builder.Gte(q => q.Difficulty, req.DifficultyMin.Value));
        if (req.DifficultyMax is not null)
            filters.Add(builder.Lte(q => q.Difficulty, req.DifficultyMax.Value));

        var filter = filters.Count == 0 ? builder.Empty : builder.And(filters);
        var limit = Math.Clamp(req.Limit, 1, 80);
        var questions = await _col.Find(filter).Limit(limit).ToListAsync();
        return questions.OrderBy(_ => Guid.NewGuid()).ToList();
    }

    public async Task<Question?> GetByIdAsync(string id) =>
        await _col.Find(q => q.Id == id).FirstOrDefaultAsync();

    public async Task<Question?> CreateAsync(UpsertQuestionRequest req)
    {
        var question = FromRequest(req);
        question.CreatedUtc = DateTime.UtcNow;
        question.UpdatedUtc = DateTime.UtcNow;

        try
        {
            await _col.InsertOneAsync(question);
            return question;
        }
        catch (MongoWriteException ex) when (ex.WriteError.Category == ServerErrorCategory.DuplicateKey)
        {
            return null;
        }
    }

    public async Task<Question?> UpdateAsync(string id, UpsertQuestionRequest req)
    {
        var question = FromRequest(req);
        question.Id = id;
        question.UpdatedUtc = DateTime.UtcNow;

        var existing = await GetByIdAsync(id);
        if (existing is null)
            return null;

        question.CreatedUtc = existing.CreatedUtc;
        var result = await _col.ReplaceOneAsync(q => q.Id == id, question);
        return result.MatchedCount == 0 ? null : question;
    }

    public async Task<int> UpsertManyByExternalIdAsync(IEnumerable<UpsertQuestionRequest> requests)
    {
        var count = 0;
        foreach (var req in requests)
        {
            var question = FromRequest(req);
            question.UpdatedUtc = DateTime.UtcNow;

            var filter = Builders<Question>.Filter.Eq(q => q.ExternalId, question.ExternalId);
            var existing = await _col.Find(filter).FirstOrDefaultAsync();
            if (existing is not null)
            {
                question.Id = existing.Id;
                question.CreatedUtc = existing.CreatedUtc;
            }
            else
            {
                question.CreatedUtc = DateTime.UtcNow;
            }

            await _col.ReplaceOneAsync(filter, question, new ReplaceOptions { IsUpsert = true });
            count++;
        }

        return count;
    }

    public async Task<bool> DeleteAsync(string id)
    {
        var result = await _col.DeleteOneAsync(q => q.Id == id);
        return result.DeletedCount > 0;
    }

    public async Task<List<string>> GetSystemsAsync(bool includeUnpublished)
    {
        var filter = includeUnpublished
            ? Builders<Question>.Filter.Empty
            : Builders<Question>.Filter.Eq(q => q.IsPublished, true);
        var systems = await _col.Distinct(q => q.System, filter).ToListAsync();
        return systems.Where(s => !string.IsNullOrWhiteSpace(s)).OrderBy(s => s).ToList();
    }

    private static Question FromRequest(UpsertQuestionRequest req) => new()
    {
        ExternalId = req.ExternalId.Trim(),
        Stem = req.Stem.Trim(),
        LeadIn = req.LeadIn.Trim(),
        Choices = req.Choices,
        CorrectChoiceId = req.CorrectChoiceId.Trim(),
        Explanation = req.Explanation.Trim(),
        ChoiceExplanations = req.ChoiceExplanations,
        Discipline = req.Discipline.Trim(),
        System = req.System.Trim(),
        Difficulty = req.Difficulty,
        Labs = req.Labs,
        Media = req.Media,
        Tags = req.Tags.Where(t => !string.IsNullOrWhiteSpace(t)).Select(t => t.Trim()).Distinct(StringComparer.OrdinalIgnoreCase).ToList(),
        IsPublished = req.IsPublished
    };
}