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这与我之前的问题有关：Coalesce fields in a .net MVC 4 model without getting "Only initializers, entity members, and entity navigation properties are supported" from LINQ

将静态 LINQ 表达式添加到我的模型后，我可以在直接从该模型类的可枚举中进行选择时在方法样式的 LINQ 语句中使用该表达式（如上述问题的选定答案和下面的示例）。现在我必须尝试从相关模型中使用该表达式，并且再次超出了我对 LINQ 和 EF 的了解。

涉及的简化模型有：

//The model class for Items for sale in the system
public class Item
{
    [Key, Column(Order = 0)]
    public String CompanyId { get; set; }

    [Key, Column(Order = 1)]
    public String ItemId { get; set; }

    public Int32 CategoryId { get; set; }

    [ForeignKey("CategoryId")]
    public virtual GlobalCategory GlobalCategory { get; set; }

    [ForeignKey("CompanyId, CategoryId")]
    public virtual CompanyCategory CompanyCategory { get; set; }

    //LINQ Expression to coalesce a company specific category alias
    //with the default global category when no alias exists
    public static Expression<Func<Item, String>> linqCategoryName
    {
        get
        {
            return i => (i.CompanyCategory.CategoryAlias == null || 
                         i.CompanyCategory.CategoryAlias == "") ?
                         i.GlobalCategory.CategoryName :
                         i.CompanyCategory.CategoryAlias;
        }
    }
}

和

//The model class for Item Relationships, used to relate two items 
//(a "Parent Item" and "Related Item") such as one is a supply for to the other
public class ItemRelationship
{
    [Key]
    public Int32 RelationshipId { get; set; }

    public Int32 RelationshipTypeId { get; set; }

    [ForeignKey("RelationshipTypeId")]
    public virtual RelationshipType RelationshipType { get; set; }

    public String CompanyId{ get; set; }

    public String ItemId{ get; set; }

    [ForeignKey("CompanyId, ItemId")]
    public virtual Item ParentItem { get; set; }

    public String RelatedCompanyId{ get; set; }

    public String RelatedItemId{ get; set; }

    [ForeignKey("RelatedCompanyId, RelatedItemId")]
    public virtual Item RelatedItem { get; set; }
}

Item 类的 LINQ Expression 成员可以成功地与这种 LINQ 语句一起使用：

var categories = usscodb.Items.Where(i => i.CompanyId = siteCompanyId)
                               .OrderBy(Item.linqCategoryName)
                               .Select(Item.linqCategoryName)
                               .Distinct();

但是，我不确定如何在以下情况下使用静态 Item.linqCategoryName：

var categories = db.ItemRelationships
                .Where(ir => ir.RelationshipType.RelationshipTypeName == "Supply"
                    && ir.ParentItem.Active
                    && ir.RelatedItem.Active
                    && ir.ParentItem.CompanyId == siteCompanyId
                    && ir.RelatedItem.CompanyId == siteCompanyId)

                //the linqCategoryName member is not available for use this way
                .Select(ir.ParentItem.linqCategoryName)
                .Distinct();

我想我在这里很近。我已经尝试使用父项的键将项目关系显式加入到项目，然后尝试使用静态 LINQ 表达式，但我认为由于项目关系中存在两个项目，LINQ 无法确定我想要的。我不确定如何明确告诉 LINQ 我想从哪个 Item 使用静态方法。

4

1 回答 1

1

试试这个：

.Select(ir => ir.ParentItem)
.Select(Item.linqCategoryName)
于 2012-11-30T17:38:03.233 回答