实体框架4.1。更新许多到许多关系。这是正确的方式？ [英] Entity Framework 4.1. Updating many-to-many relationships. Is this the right way?

问题描述

以下作品中的code，然而，我怀疑我失去了一些东西。是否有一个'好'的方式？

The code below works, however, I suspect that I'm missing something. Is there a 'better' way?

    private void UpdateNew(MarketProduct marketproduct)
    {
        context.MarketCategories.Load();
        MarketProduct dbProd = context.MarketProducts.Find(marketproduct.Id);
        dbProd.Categories.Clear();
        foreach (var c in marketproduct.Categories ?? Enumerable.Empty<MarketCategory>())
        {
            var cc = context.MarketCategories.Find(c.Id);
            dbProd.Categories.Add(cc);
        }
        context.Entry(dbProd).CurrentValues.SetValues(marketproduct);
    }

我认为这将是可能做到这一点，而无需使用查找

推荐答案

您有三个数据库查询：1） context.MarketCategories.Load（）（希望类别表小，否则这将是一个不走的，因为它加载整个表到内存中），2） ...找 3） dbProd .Categories.Clear（）：这里必须涉及延迟加载，否则这会崩溃，因为 dbProd.Categories 是空。

You have three database queries: 1) context.MarketCategories.Load() (hopefully the category table is small, otherwise this would be a no-go as it loads the whole table into memory), 2) ...Find and 3) dbProd.Categories.Clear(): Here must be a lazy loading involved, otherwise this would crash because dbProd.Categories would be null.

这是另一种与单个数据库查询来更新如下：

An alternative to update with a single database query is the following:

private void UpdateNew(MarketProduct marketproduct)
{
    MarketProduct dbProd = context.MarketProducts
        .Include(p => p.Categories)
        .Single(p => p.Id == marketproduct.Id);

    var categories = marketproduct.Categories 
                     ?? Enumerable.Empty<MarketCategory>();
    foreach (var category in categories)
    {
        if (!dbProd.Categories.Any(c => c.Id == category.Id))
        {
            // means: category is new
            context.MarketCategories.Attach(category);
            dbProd.Categories.Add(category);
        }
    }
    foreach (var category in dbProd.Categories.ToList())
    {
        if (!categories.Any(c => c.Id == category.Id))
            // means: category has been removed
            dbProd.Categories.Remove(category);
    }

    context.Entry(dbProd).CurrentValues.SetValues(marketproduct);

    // context.SaveChanges() somewhere
}

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