PurchaseProduct.cs

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

    public PurchaseProductHandler(SalesDbContext dbContext, IProductQuery productQuery)
    {
        _dbContext = dbContext;
        _productQuery = productQuery;
    }

    public async Task<Unit> Handle(PurchaseProduct request, CancellationToken cancellationToken)
    {
        var product = await _productQuery.GetProduct(request.ProductId);

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

        return Unit.Value;
    }
}