[英]Entity Framework adding existing Child to a new Parent on a many to many relation
When you want to add an existing Child to a new Parent (1 to 1 or 1-n relation), working with code first, you can just define inside Parent the Child and ChileId and EF automap that id to the child. 当您想将现有的子级添加到新的父级(1对1或1-n关系)时,首先使用代码,您可以在父级内部定义Child以及ChileId和EF自动将ID映射到该子级。 Is there any way to do the same on a many to many relation??
有什么办法可以在多对多关系上做同样的事情?
Parent
{
int ChildId {get;set;}
aClass Child {get;set;}
}
architecture data: Entity Framework, code first. 体系结构数据:实体框架,代码优先。 Backend webapi/restfull disconnected UI maps a ParentData to ParentEntity Child collection would be something like "countires", so I dont want to add new ones, but just relate many countires to the Parent.
后端webapi / restfull断开连接的UI将ParentData映射到ParentEntity Child集合就像是“国家”,因此我不想添加新的国家,而只是将许多国家与父级相关联。 There is a multiselect drop-down on the UI, so you can check/uncheck the countries.
用户界面上有一个多选下拉菜单,因此您可以选中/取消选中国家。
eg 例如
Parent related with USA, UK 父母与美国,英国有关
Then at UI someone also checks ESP the 3 will be related to the Parent 然后在用户界面上也有人检查ESP 3是否与父级有关
In many-to-many, it's not as easy to work with ID instead of whole objects. 在很多对很多情况下,使用ID而不是使用整个对象并不是那么容易。
Consider the following: 考虑以下:
class Parent
{
public Parent()
{
Children = new List<Child>();
}
public int Id {get;set;}
public ICollection<Child> Children { get; set; }
}
class Child
{
public Child()
{
Parents = new List<Parent>();
}
public int Id {get;set;}
public ICollection<Parent> Parents { get; set; }
}
If the existing child is not loaded (and not wanted to be pre-loaded), you can attach a child entry with ID only to establish the relation: 如果未加载现有子项(并且不希望对其进行预加载),则可以附加一个ID为ID的子项以建立关系:
int existingChildId; // some value
var childPlaceholder = new Child { Id = existingChildId };
db.Children.Attach(childPlaceholder);
var newParent = new Parent();
newParent.Children.Add(childPlaceholder);
db.Parents.Add(newParent);
db.SaveChanges();
If you don't know whether the child is already loaded in the context and you still want to avoid a database query to load it, inspect the local
entries: 如果您不知道该子项是否已经在上下文中加载,并且仍然希望避免数据库查询来加载它,请检查
local
条目:
int existingChildId; // some value
var childPlaceholder = db.Children.Local.FirstOrDefault(x => x.Id == existingChildId) ??
db.Children.Attach(new Child { Id = existingChildId });
// placeholder is ready to use
using (var context = new YourContext())
{
var mathClass= new Class { Name = "Math" };
Student student1 = context.Students.FirstOrDefault(s => s.Name == "Alice");
Student student2 = context.Students.FirstOrDefault(s => s.Name == "Bob");
mathClass.Students.Add(student1);
mathClass.Students.Add(student2);
context.AddToClasses(mathClass);
context.SaveChanges();
}
may be this could help you. 也许这可以帮助您。
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