LINQ to Entities查询相关实体

Question

I have 3 EF ObjectSets: Parents, Childrens and Pets, with 1-to-many associations (Parent can have mayn Children and Children can have many Pets). For a given Parent, I need to get all the Pets that this Parent's all Children own.

I'm concerned if this query is correct or can it be made more efficient:

Dim query = From par In context.Parents
            From child In par.Childrens
            From pet In child.Pets
            Where par.parent_id = 1
            Select pet

Answer 1

This query is correct although its a cross-join - is simply the Cartesian product of two sets -.

You can explicitly specify a join based on equality condition.

var query = from par  in context.Parents
            join child  in context.Childrens on child.ParentID equals par.ID
            join pet  in context.Products on child.ID equals pet.ChildID
            Where par.parent_id = 1
            select new { par.Name, par.Id };