使用部分加载的集合获取实体框架对象

问题描述

  class Farm {
 string owner; 
列表<动物>动物; 
 DateTime StartDate; 
} 
 
 class Animal {
 string Name; 
 DateTime DOB; 
} 
  

问题：

我想选择 我已经尝试了以下内容：

Try1 ：

  //这仍然显示每个农场的所有动物，如果有是至少一个
 //动物与所需的DOB 
 
 var x = context.Farm.Where（y => y.StartDate> = myDate 
&& ; y.Animal.Any（z => z.DOB> = otherDate）
）.Include（Animal）; 
  

Try2 ：

  //我将Farm类子类化，因为我不能直接从Entity Framework实例化类
 //，这应该是我的返回类型。 
 class Temp：Farm {} 
 
 var x = context.Farm.Where（y => y.StartDate> = myDate）.Include（Animal）
 。选择（z => new Temp（）{
 owner = z.owner，
 animals = new TrackableCollection< Animal>（）{z.animals.Where（y => y.DOB> ; = newDate）.SingleOrDefault（）}）; 
 
 //这里的一些事情：
 // 1：我实例化了一个新的TrackableCollection，因为这是Animal的集合
 //类型在Entity Framework中。 
 // 2：由于某种原因，这仍然无法工作，如果我使用这种方法，农场中的动物列表
 //附带0个元素。 
  

Try3 ：

阅读完毕后：

  var x =（from farm in ctx.Farm 
 from animal in farm.Animal 
其中animal.DOB => newDate 
 select new {farm，animal}）AsEnumerable（）选择（x => x.farm）.Distinct（）ToList（）; 
 //我不知道这是如何工作的，但它是... 
  

任何人小心解释上述如何工作？

基本上，查询是选择父实体和由实体过滤的子实体，通过Relationship Fixup知道实体框架所选择的孩子与所选择的父母相关联，因此它们也被添加到父集合中。我看到这是一个奇怪的解决方案，但它的确有效。

- Andrei D。

解决方案

以下作为两个单独的查询：

  var x =（from farm in ctx.Farm 
 from animal in farm。 Animal 
其中，animal.DOB => newDate 
 select new {farm，animal}）AsEnumerable（）选择（x => x.farm）.Distinct（）ToList（）; 
  

分解：

  //给我所有农场
从农场ctx.Farm 
 
 //给我农场一个DOB大于或等于newDate 
的动物动物在farm.Animal 
其中，animal.DOB => newDate 
 
 //选择两者，以便在执行期间都不会从查询中被排除
选择新的{farm，animal}）
  

在执行时，查询将仅包含上述内容的数据，因此结果将包含每个 Farm ，包括过滤的动物。

Distinct 过滤副本。

I have an entity Framework model with the following:

class Farm{
    string owner;
    List<Animal> animals;
    DateTime StartDate;
}

class Animal{
    string Name;
    DateTime DOB;
}

Problem:

I would like to select a collection of farms whose start date is >= 2013/01/01 along with it's animals, but also filtered by DOB >= 2013/06/01.

I've tried the following:

Try1:

//This still shows all animals from each farm, if there is at least one
//animal with the required DOB

var x = context.Farm.Where(y => y.StartDate >= myDate
                           && y.Animal.Any(z => z.DOB >= otherDate)
                          ).Include("Animal");

Try2:

//I subclassed the Farm class because i cant instantiate the class
//from Entity Framework directly, and that should be my return type.
class Temp:Farm{}

var x = context.Farm.Where(y => y.StartDate >= myDate).Include("Animal")
        .Select(z => new Temp(){
                    owner = z.owner,
                    animals = new TrackableCollection<Animal>(){ z.animals.Where(y => y.DOB >= newDate).SingleOrDefault() });

//Couple of things here:
//1: I instantiated a new TrackableCollection because thats what the collection
//type of Animal is inside Entity Framework.
//2: This still doesnt work for some reason, if i use this approach, the list
//of animals in the farm comes with 0 elements.

Try3:

After reading this: Ef-query-with-conditional-include

var x = (from farm in ctx.Farm
        from animal in farm.Animal
        where animal.DOB => newDate
        select new{farm, animal}).AsEnumerable().Select(x=> x.farm).Distinct().ToList();
//I have no idea how this works, but it does...

Anyone care to explain how the above works?

Basically the query is selecting the parent entity and the child entity filtered by the required parameters, and entity framework through "Relationship Fixup" knows that the selected children are associated with the selected parents, so they get added to the parent collection as well. I see it kind of a hacky solution, but it works indeed.

--Andrei D.

解决方案

Look at the following as two separate queries:

var x = (from farm in ctx.Farm
        from animal in farm.Animal
        where animal.DOB => newDate
        select new{farm, animal}).AsEnumerable().Select(x=> x.farm).Distinct().ToList();

Broken out:

//Give me all farms
from farm in ctx.Farm

//Give me farms with animals with a DOB greater or equal to newDate
from animal in farm.Animal
where animal.DOB => newDate

//Select both so that neither are discluded from the query during execution
select new{farm, animal})

At the point of execution, the query will only contain data from whats above, so the result will contain two of every Farm, including the filtered Animals.

The Distinct filters the duplicates.

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