PurchaseProduct.cs

public class PurchaseProductHandler : IRequestHandler<PurchaseProduct>
{
    private readonly SalesDbContext _dbContext;

    public PurchaseProductHandler(SalesDbContext dbContext)
    {
        _dbContext = dbContext;
    }

    public async Task<Unit> Handle(PurchaseProduct request, CancellationToken cancellationToken)
    {
        var price = await _dbContext.Products
            .Where(x => x.Id == request.ProductId)
            .Select(x => x.Price)
            .SingleAsync();

        var order = new Order
        {
            OrderId = Guid.NewGuid(),
            Items = new List<OrderItem>
            {
                new OrderItem
                {
                    ProductId = request.ProductId,
                    Price = price
                }
            }
        };
        await _dbContext.Orders.AddAsync(order, cancellationToken);
        await _dbContext.SaveChangesAsync();

        return Unit.Value;
    }
}