EF两次击中数据库..我在做什么错？

Question

I have the following code:

    private static List<ProxyServer> proxyServers = new List<ProxyServer>();

    private static IEnumerable<Product> products = new List<Product>();

    private static CharonTestEntities3 context = new CharonTestEntities3();
    public Scrape()
    {

        proxyServers = context.ProxyServers.ToList();
        products = context.Products;
    }

    private ProxyServer getProxy(int countryID)
    {
        //TODO: random proxy based on last usage (< 40 seconds)
        return proxyServers.FirstOrDefault();


    }
    private bool getProducts(string asin)
    {
        var product = products.First(x => x.Asin == asin);

        if (products.First().ProductID > 0)
        {
            return true;
        }

        return false;

    }
    public void DoScrape(SearchCriteria criteria)
    {

        getProducts("A1234543345");
    }

When i run getProducts(); it hits the database to look up the product but I thought it would use my products list<>.. can anyone expain why my initial scrape = new Scrape() doesn't cache the products list to the products object?

Answer 1

Because code products = context.Products; is not a list but querable object. If you change your code to products = context.Products.ToList(); it will query SQL for all data once.

Moreover products.First(x => x.Asin == asin) is something completely different to products.First() and so you don't have a cache, but querable object it hits database twice

Answer 2

Your products list is of type IEnumerable() .

It hits the DB on getProducts because that is the only time it really needs the data.

To resolve this, in Scrape() , add .ToList() to context.Products

Answer 3

The thing is that the property Products on the context instance is implementing IEnumerable and therefore when you are associating it to your field products you are only casting the querable object to IEnumerable.

You will keep seeing the First and probably any other extension method that IEnumerable gives you if you use context.Products directly (please try).

So, you need to cast it to List to force it to perform the query on the ctor (the same as you are doing for ProxyServers ).

More:

1. You are doing a First without Default, it might throw an exception.
2. I'm not sure why you are validating if the first product on products has ProductID > 0.