LINQ 表达式无法翻译 - C#

Question

我写了一个简单的查询（问题是当我尝试在Address下面设置时ProductCode）：

 var query = _connectDBContext.Products
                            .Join(_context.CustomerRelations,
                                Product => Product.ForeignKeyId,
                                CustomerRelation => CustomerRelation.CompanyId,
                                (Product, CustomerRelation) => new { Product, CustomerRelation })
                            .Select(x => new ProductDto
                            {
                                Title = x.Product.Title,
                                ProductCode = x.Product.Code,
                                Address = Map(x.Product.Addresses.OrderBy(x=> x.CreatedDate).FirstOrDefault()),
                                //Address = new AddressDTO // THIS WORKS BUT LOOKS UGLY :(
                                //{
                                //    Address = x.Product.Addresses.OrderBy(x => x.CreatedDate).FirstOrDefault().Address,
                                //    Country = x.Product.Addresses.OrderBy(x => x.CreatedDate).FirstOrDefault().Country,
                                //    Zip = x.Product.Addresses.OrderBy(x => x.CreatedDate).FirstOrDefault().Zip,
                                //},
                               
                            })
                            .Where(x => x.Id == x.CustomerRelationId);

           // Rest of the code
        }

private AddressDTO Map(Address address)
{
    return new AddressDTO
    {
        Address = address.Address,
        Country = address.Country,
        Zip = address.Zip,

    };
}

上面的代码在这一行中断：

Address = Map(x.Product.Addresses.OrderBy(x=> x.CreatedDate).FirstOrDefault()),

它说无法翻译 linq 并建议我重写查询..

但是这里注释的代码几乎可以完成相同的工作，所以如果我在Select 中删除调用Map方法，并且如果我在Select 中取消注释此代码，一切都会起作用，但我想摆脱这个 -为每个道具写太多，我想订购一次，然后我想使用它..但不幸的是我不知道如何..：OrderBy

//Address = new AddressDTO 
//{
//    Address = x.Product.Addresses.OrderBy(x => x.CreatedDate).FirstOrDefault().Address,
//    Country = x.Product.Addresses.OrderBy(x => x.CreatedDate).FirstOrDefault().Country,
//    Zip = x.Product.Addresses.OrderBy(x => x.CreatedDate).FirstOrDefault().Zip,
//},

提前致谢，干杯

Answer 1

大概 Entity Framework 正在将此代码转换为 SQL，而Map()SQL 不知道您的自定义方法。幸运的是，该方法的作用不大，因此您应该能够将其功能直接移到查询中。

您可以使用.Select()将集合投影到新类型中（就像您已经为构建您的 所做的那样ProductDto）。

例如：

//...
Address = x.Product.Addresses.OrderBy(x=> x.CreatedDate)
                             .Select(x=> new AddressDTO
                             {
                                 Address = x.Address,
                                 Country = x.Country,
                                 Zip = x.Zip
                             })
                             .FirstOrDefault()
//...