当不存在显式关系时，使用实体框架请求父子对象集合

Question

I have 2 classes, see below They are POCO classes which we use to access the database, we use code first approach and configure all mapping between entities and DB tables explicitly, using ApplicationEntityConfiguration.

 public class Group
{
    public string Code { get; set; }
    public string Name { get; set; }
    public virtual ICollection<Item> Items { get; set; }
}

public class Item
{
    public string Code { get; set; }
    public string Name { get; set; }
    public virtual Group ParentGroup { get; set; }
}

The relatioship beteen Groups and Childs is not explicit. There is no foreign key. Logically relationship is defined by Code, Group should contain all items which have codes staring with the same code as parent group

Context.Items.Where(x => x.Code.StartsWith(parentGroup.Code));

I need to return to caller method a collection of all Groups in the database with all childs for each group. When requesting the data I can request Context.Groups and for each Group request Items using Where. I have more then 50 groups and more then 1000 items in the database So such manual approach would result in more then 50 queries to the database and will have performance impact

More optimal solution would be to request all Group in one collection, then all Item in another collection and then when all those data in memory, fill a collection of Items for each group... then it will not hit DB a lot.

So, while I able to achive the goal in general, I wonder if there is a more elegant solution where we can declarativly tell Enity Framework that relationship between groups and items is established using the expression .Where(x => x.Code.StartsWith(parentGroup.Code)) and then once it is declared, developer can use Include() when he wants to Load groups and items together or rely on Lazy loading that can load child items when they will be accessed

Answer 1

There is no better way to do this than your second approach. This is because Entity Framework is only able to load collections that have a foreign key relationship to the PK of the parent. There is no support for associations to unique indexes yet.

Likewise, the process you're referring to, relationship fixup , can only work when the association is mapped. But in your case it can't be mapped, so EF can't populate the collections itself.