英文:
Queryable.Join with the WHERE clause using inner table?
问题
我试图使用Queryable.Join方法将对象列表连接在一起，但我想针对内部表使用WHERE子句。在这种情况下，我无法颠倒连接表的顺序。当我添加WHERE子句时，它只“看到”外部表中的字段。以下是重现我的问题的示例代码：
using System;
using System.Collections.Generic;
using System.Linq;
namespace JoinsWithIQueriable
{
internal class Program
{
class Person
{
public string Name { get; set; }
public int Id { get; set; }
}
class Pet
{
public string Name { get; set; }
public int WeightInLbs { get; set; }
public Person Owner { get; set; }
public Species PetSpecies { get; set; }
}
class Species
{
public string Name { get; set; }
}
static void Main(string[] args)
{
Species dog = new Species { Name = "Dog" };
Species cat = new Species { Name = "Cat" };
Person magnus = new Person { Name = "Hedlund, Magnus", Id = 1 };
Person terry = new Person { Name = "Adams, Terry", Id = 2 };
Person charlotte = new Person { Name = "Weiss, Charlotte", Id = 3 };
Person alergictopets = new Person { Name = "Scratchy, Itchy", Id = 4 };
Pet barley = new Pet { Name = "Barley", WeightInLbs = 10, Owner = terry, PetSpecies = cat };
Pet boots = new Pet { Name = "Boots", WeightInLbs = 11, Owner = terry, PetSpecies = cat };
Pet whiskers = new Pet { Name = "Whiskers", WeightInLbs = 12, Owner = charlotte, PetSpecies = cat };
Pet daisy = new Pet { Name = "Daisy", WeightInLbs = 40, Owner = magnus, PetSpecies = dog };
List<Person> people = new List<Person> { magnus, terry, charlotte, alergictopets };
List<Pet> pets = new List<Pet> { barley, boots, whiskers, daisy };
List<Species> species = new List<Species> { dog, cat };
// 使用Queryable.Join方法将Person对象列表和Pet对象列表连接，
// 创建一个包含宠物名称和拥有宠物的人员名称的匿名类型的列表。
Console.WriteLine("query");
var query = people.AsQueryable().Join(pets,
person => person,
pet => pet.Owner,
(person, pet) =>
new { OwnerName = person.Name, Pet = pet.Name, Weight = pet.WeightInLbs });
foreach (var obj in query)
{
Console.WriteLine(
"Owner {0} - Pet {1} - Pet weight {2} ",
obj.OwnerName,
obj.Pet,
obj.Weight.ToString());
}
/*
此代码生成以下输出：
Owner Hedlund, Magnus - Pet Daisy - Pet weight 40
Owner Adams, Terry - Pet Barley - Pet weight 10
Owner Adams, Terry - Pet Boots - Pet weight 11
Owner Weiss, Charlotte - Pet Whiskers - Pet weight 12
*/
Console.WriteLine("querywithwhere");
// 想要筛选体重小于12磅的宠物
var querywithwhere = people.AsQueryable()
.Join(pets,
person => person,
pet => pet.Owner,
(person, pet) =>
new { OwnerName = person.Name, Pet = pet.Name, Weight = pet.WeightInLbs });
foreach (var obj in querywithwhere)
{
Console.WriteLine(
"Owner {0} - Pet {1} - Pet weight {2} ",
obj.OwnerName,
obj.Pet,
obj.Weight.ToString());
}
Console.WriteLine("Press any key to continue...");
Console.ReadLine();
}
}
}
希望这对你有所帮助。
英文:
I'm trying to use the Queryable.Join method to join Lists of objects together, but I want to use the WHERE clause against the inner table. I cannot reverse the order of the join tables in this case. When I add a WHERE clause it only "sees" the fields in the outer table. Here's a sample that reproduces my problem:
using System;
using System.Collections.Generic;
using System.Linq;
namespace JoinsWithIQueriable
{
internal class Program
{
class Person
{
public string Name { get; set; }
public int Id { get; set; }
}
class Pet
{
public string Name { get; set; }
public int WeightInLbs { get; set; }
public Person Owner { get; set; }
public Species PetSpecies { get; set; }
}
class Species
{
public string Name { get; set; }
}
static void Main(string[] args)
{
Species dog = new Species { Name = "Dog" };
Species cat = new Species { Name = "Cat" };
Person magnus = new Person { Name = "Hedlund, Magnus", Id = 1};
Person terry = new Person { Name = "Adams, Terry", Id = 2 };
Person charlotte = new Person { Name = "Weiss, Charlotte", Id = 3 };
Person alergictopets = new Person { Name = "Scratchy, Itchy", Id = 4 };
Pet barley = new Pet { Name = "Barley", WeightInLbs = 10, Owner = terry, PetSpecies = cat };
Pet boots = new Pet { Name = "Boots", WeightInLbs = 11, Owner = terry, PetSpecies = cat };
Pet whiskers = new Pet { Name = "Whiskers", WeightInLbs = 12, Owner = charlotte, PetSpecies = cat };
Pet daisy = new Pet { Name = "Daisy", WeightInLbs = 40, Owner = magnus, PetSpecies = dog};
List<Person> people = new List<Person> { magnus, terry, charlotte, alergictopets };
List<Pet> pets = new List<Pet> { barley, boots, whiskers, daisy };
List<Species> species = new List<Species> { dog, cat};
// Join the list of Person objects and the list of Pet objects
// to create a list of person-pet pairs where each element is
// an anonymous type that contains the name of pet and the name
// of the person that owns the pet.
Console.WriteLine("query");
var query = people.AsQueryable().Join(pets,
person => person,
pet => pet.Owner,
(person, pet) =>
new { OwnerName = person.Name, Pet = pet.Name, Weight = pet.WeightInLbs });
foreach (var obj in query)
{
Console.WriteLine(
"Owner {0} - Pet {1} - Pet weight {2} ",
obj.OwnerName,
obj.Pet,
obj.Weight.ToString());
}
/*
This code produces the following output:
Owner Hedlund, Magnus - Pet Daisy - Pet weight 40
Owner Adams, Terry - Pet Barley - Pet weight 10
Owner Adams, Terry - Pet Boots - Pet weight 11
Owner Weiss, Charlotte - Pet Whiskers - Pet weight 12
*/
Console.WriteLine("querywithwhere");
//Want to filter for pets that weight less than 12 lbs
var querywithwhere = people.AsQueryable()
.Join(pets,
person => person,
pet => pet.Owner,
(person, pet) =>
new { OwnerName = person.Name, Pet = pet.Name, Weight = pet.WeightInLbs });
foreach (var obj in querywithwhere)
{
Console.WriteLine(
"Owner {0} - Pet {1} - Pet weight {2} ",
obj.OwnerName,
obj.Pet,
obj.Weight.ToString());
}
Console.WriteLine("Press any key to continue...");
Console.ReadLine();
}
}
}
答案1
得分: 1
要么你可以在与 people 连接之前使用包含记录 WeightInLbs < 12 的方式筛选 pets。
var querywithwhere = people.AsQueryable()
.Join(pets.Where(x => x.WeightInLbs < 12),
person => person,
pet => pet.Owner,
(person, pet) =>
new { OwnerName = person.Name, Pet = pet.Name, Weight = pet.WeightInLbs });
或者你可以在连接后使用 .Where() 子句筛选记录。
var querywithwhere = people.AsQueryable()
.Join(pets,
person => person,
pet => pet.Owner,
(person, pet) =>
new { OwnerName = person.Name, Pet = pet.Name, Weight = pet.WeightInLbs })
.Where(x => x.Weight < 12);
英文:
Either you can filter the pets with containing the record with WeightInLbs < 12 before join with people
var querywithwhere = people.AsQueryable()
.Join(pets.Where(x => x.WeightInLbs < 12),
person => person,
pet => pet.Owner,
(person, pet) =>
new { OwnerName = person.Name, Pet = pet.Name, Weight = pet.WeightInLbs });
Or you can filter the record after joining with .Where() clause.
var querywithwhere = people.AsQueryable()
.Join(pets,
person => person,
pet => pet.Owner,
(person, pet) =>
new { OwnerName = person.Name, Pet = pet.Name, Weight = pet.WeightInLbs })
.Where(x => x.Weight < 12);
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