Dapper MultiMap不适用于NULL值的splitOn [英] Dapper MultiMap doesn't work with splitOn with NULL value

问题描述

我在dapper中遇到MultiMaps问题，试图在包含 NULL 的列上拆分。 Dapper似乎不实例化对象，并且我的映射函数收到 null 而不是对象。

I have a problem with MultiMaps in dapper trying to split on column that contains NULL. Dapper seems not to instantiate object and my mapping function receives null instead of object.

这是我的新测试： / p>

Here's my new test:

    class Product
    {
        public int Id { get; set; }
        public string Name { get; set; }
        public Category Category { get; set; }
    }
    class Category
    {
        public int Id { get; set; }
        public string Name { get; set; }
        public string Description { get; set; }
    }
    public void TestMultiMapWithSplitWithNullValue()
    {
        var sql = @"select 1 as id, 'abc' as name, NULL as description, 'def' as name";
        var product = connection.Query<Product, Category, Product>(sql, (prod, cat) =>
        {
            prod.Category = cat;
            return prod;
        }, splitOn: "description").First();
        // assertions
        product.Id.IsEqualTo(1);
        product.Name.IsEqualTo("abc");
        product.Category.IsNotNull();
        product.Category.Id.IsEqualTo(0);
        product.Category.Name.IsEqualTo("def");
        product.Category.Description.IsNull();
    }

失败的行是 product。 Category.IsNotNull（）; ，因为传递给映射函数的 cat 为 null 。

The line that fails is product.Category.IsNotNull(); due to the fact that cat passed to mapping function is null.

我还将此方法添加到了断言类：

I've also added this method to Assert class:

public static void IsNotNull(this object obj)
{
    if (obj == null)
    {
        throw new ApplicationException("Expected not null");
    }
}

推荐答案

此是按设计，尽管我可以重新考虑。

This is "by-design" though I would be ok to revisit it.

特别是这种行为可以帮助左联接。以此为例：

In particular this behaviour is there to help with left joins. Take this for example:

cnn.Query<Car,Driver>("select * from Cars c left join Drivers on c.Id = CarId",
   (c,d) => {c.Driver = d; return c;}) 

问题是，如果我们允许空白创建 Driver 对象，则每个 Car 将具有 Driver ，即使连接失败。

Trouble is that if we allow a "blanket" creation of a Driver object, every Car is going to have a Driver even ones where the join failed.

要解决此问题，我们可以扫描被拆分的整个段，并在映射<$之前确保所有值均为 NULL c $ c> NULL 对象。这对多重映射器的性能影响很小。

To work around we could scan the entire segment being split and ensure ALL values are NULL before mapping a NULL object. This will have a very minor perf impact on the multi mapper.

要解决此问题，您可以插入代理列：

To workaround for your case, you could insert a surrogate column:

var sql = @"select 1 as id, 'abc' as name, '' as split, 
            NULL as description, 'def' as name";
    var product = connection.Query<Product, Category, Product>(sql, (prod, cat) =>
    {
        prod.Category = cat;
        return prod;
    }, splitOn: "split").First();

这篇关于Dapper MultiMap不适用于NULL值的splitOn的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！