using MongoDB.Bson;
using MongoDB.Bson.Serialization.Attributes;
using MongoDB.Bson.Serialization.IdGenerators;
using MongoDB.Driver;
using Xunit;

namespace MongoDB_Projection_Issue;

public class Program
{
	// Configuration values
	private const string connectionString = "{Insert Connection String}";
	private const string dbName = "Test";
	private const string collectionName = "Test";
	
	public static async Task Main()
	{
		// Connect to database.
		var url = new MongoUrl(connectionString);
		var client = new MongoClient(url);

		// Test projection on documents with ObjectId. (Fails; all values are empty ObjectIds)
		await TestObjectId(client);
		// Test projection on documents with string ID. (Fails; all values are null)
		await TestStringId(client);
	}

	private static async Task TestObjectId(MongoClient client)
	{
		var collection = client.GetDatabase(dbName).GetCollection<TestModelWithObjectId>(collectionName);
	
		// Create test documents.
		var docs = Enumerable.Range(1, 5).Select(_ => new TestModelWithObjectId());

		// Delete any existing documents
		await collection.DeleteManyAsync(x => true);

		// Insert documents in collection.
		await collection.InsertManyAsync(docs);

		// Projection to get IDs of documents in collection.
		var projection = collection.Find(_ => true)
		                           .Project(x => x.Id);
		var ids = await projection.ToListAsync();
		var results = await collection.Find(x => true).ToListAsync();

		// Write projection to the console for validation.
		Console.WriteLine(projection.ToString());

		// Assert that all results are not null.
		Assert.All(results, x => Assert.NotEqual(ObjectId.Empty, x.Id));
		Assert.All(ids, x => Assert.NotEqual(ObjectId.Empty, x));
	}

	private static async Task TestStringId(MongoClient client)
	{
		var collection = client.GetDatabase(dbName).GetCollection<TestModelWithStringId>(collectionName);
	
		// Create test documents.
		var docs = Enumerable.Range(1, 5).Select(_ => new TestModelWithStringId());

		// Delete any existing documents
		await collection.DeleteManyAsync(x => true);

		// Insert documents in collection.
		await collection.InsertManyAsync(docs);

		// Projection to get IDs of documents in collection.
		var projection = collection.Find(_ => true)
		                           .Project(x => x.Id);
		var ids = await projection.ToListAsync();
		var results = await collection.Find(x => true).ToListAsync();

		// Write projection to the console for validation.
		Console.WriteLine(projection.ToString());

		// Assert that all results are not null.
		Assert.All(results, x => Assert.NotNull(x.Id));
		Assert.All(ids, x => Assert.NotNull(x?.ToString()));
	}
}

/// <summary>
/// Document model that uses a string for the document ID.
/// </summary>
public class TestModelWithStringId
{
	/// <summary>
	/// Document ID.
	/// </summary>
	[BsonId(IdGenerator = typeof(StringObjectIdGenerator))]
	[BsonRepresentation(BsonType.ObjectId)]
	public string? Id { get; set; }

	/// <summary>
	/// Document type; here only for differentiation in results.
	/// </summary>
	[BsonElement]
	public string Type => nameof(TestModelWithStringId);
}

/// <summary>
/// Document model that uses an Object ID for the document ID.
/// </summary>
public class TestModelWithObjectId
{
	/// <summary>
	/// Document ID.
	/// </summary>
	[BsonId]
	public ObjectId Id { get; set; }

	/// <summary>
	/// Document type; here only for differentiation in results.
	/// </summary>
	[BsonElement]
	public string Type => nameof(TestModelWithObjectId);
}