/// <summary>
/// 获取给定的 <see cref="IAsyncCursorSource{BsonDocument"/> 异步游标所迭代的到的第一个 <see cref="BsonDocument"/>
/// 对象，并将该对象反序列化为给定的实体对象类型。
/// </summary>
/// <param name="source">包含数据的异步游标。</param>
/// <typeparam name="TEntity">要转换到的实体对象类型。</typeparam>
/// <returns>
/// 给定的异步游标所迭代到的第一个实体对象。如果给定的异步游标无法迭代出任何对象，返回
/// <code>default(<typeparamref name="TEntity"/>)</code>。
/// </returns>
public static async Task <TEntity> FirstEntityOrDefaultAsync <TEntity>(
this IAsyncCursorSource <BsonDocument> source)
{
Contract.NotNull(source, nameof(source));
var bsonDoc = await source.FirstOrDefaultAsync();
if (Equals(bsonDoc, default(TEntity)))
{
return(default);
public async Task <BsonDocument> ReadVersionAsync(string symbol)
{
Log.Debug("reading {symbol} version", symbol);
IAsyncCursorSource <BsonDocument> versions = this._versions.AsQueryable()
.Where(x => x ["symbol"] == symbol)
.OrderByDescending(x => x ["version"])
.Take(1);
var rtn = await versions.FirstOrDefaultAsync <BsonDocument>();
Log.Debug("read {0} version: {1}".Args(symbol, rtn));
return(rtn);
}