public async Task <long> BalanceAsync(Guid lenderId, Guid lendeeId, string code)
{
var from = await _context.Participants.FirstOrDefaultAsync(c => c.Id == lenderId);
var to = await _context.Participants.FirstOrDefaultAsync(c => c.Id == lendeeId);
var lended = _context.Entry(from).Collection(c => c.Lended).Query().Where(c => c.Lendee == to && c.Amount.Currency.Code == code).Sum(c => c.Amount.Value);
var borrowed = _context.Entry(from).Collection(c => c.Borrowed).Query().Where(c => c.Lender == to && c.Amount.Currency.Code == code).Sum(c => c.Amount.Value);
return(lended - borrowed);
}
