public async Task SellShares(ulong userId, ulong guildId, string symbol, long shares, decimal price)
{
// position guaranteed to already exist
Investment position = await _context.Investments.AsQueryable().SingleOrDefaultAsync(x => x.UserId == userId && x.GuildId == guildId && x.Symbol == symbol) ?? new Investment(userId, guildId, symbol, -shares);
position.Shares -= shares;
position.Trades.Add(new Trade(-shares, price));
await _currency.AddInvestingAsync(userId, guildId, price *shares);
await _context.SaveChangesAsync();
}
