如何使用Entity Framework从具有一对多关系的两个表中选择所有相关数据？

Question

I am struggling with Entity Framework as I would like to select everything related to one item from Item table and the other two tables Label and ItemLabel. The relationship between Item and ItemLabel tables is one to many.

I would like to write IEnumberable List method that will retrieve all the data related to an item. However, I don't know how to retrieve the all the data in the ItemLabel table.

Here's my schema:

Item Table: ItemId, Title, Description
Label Table: LabelId, Title
ItemLabel Table: ItemLabelId, ItemId, LabelId, Description

and here's my Item class in the Data Access Layer

public int ItemId { get; set; }
public string Title { get; set; }
public string Description { get; set; }

public IEnumerable<Item> GetItems(Item itemObj)

    {

        List<Item> itemList = new List<Item>();

        using (TestEntities context = new TestEntities())

        {

            itemList = (from item in context.T_Item

                        select new Item()

                        {

                            ItemId = item.IdeaId,

                            Title = item.Title,

                            Description = item.Description,

                            Labels = item.T_ItemLabel.FirstOrDefault(), <<<<<< Error

                        }).ToList();

        }

        return itemList;

    }

Please note that I am using Database First approach.

So could you please tell me how can I get all the labels related to each item I have in the Item table? Am I missing anything?

Answer 1

If you're selecting an entity type, you can just select it - you don't have to construct an object like you're doing. Simplest is var itemList = content.T_item because DbSet is also an IEnumerable but either of the following will work:

var itemList = (from item in context.T_Item select item);
var itemList = context.T_item.Select(item => item);

You can then access the Labels on each Item by just using the navigation property: var labels = itemList.First().Labels . The collections are lazy-loaded, so this involves another trip to the database. Add .Include("T_ItemLabel") to the context.T_item to fetch all the Labels in the original query.