Question

I have an entity Person, which has a related entity Hobby. This is a many to many relationship where a person can have several hobbies, and a hobby can be associated with many people. I want to create a ViewModel that allows new hobbies to be added and/or existing hobbies to be removed from a given Person entity.

The code below works, but I imagine it is rather inefficient.

var newHobbies = new List<Hobby>();

foreach (Hobby hobby in vm.Hobbies)
{
    var originalHobby = db.Hobbies.Find(hobby.HobbyID);
    newHobbies.Add(originalHobby);
}

originalPerson.Hobbies = newHobbies;

I prefer to do something like this:

var newHobbies = db.Hobbies.Where(x => vm.Hobbies.All(y => x.HobbyID == y.HobbyID)).ToList();   
originalPerson.Hobbies = newHobbies;

But I get an error:

Only primitive types or enumeration types are supported in this context.

How can I update related data without going to the database multiple times?

Answer 1

消息说你不能在LINQ-to-Entities查询中使用vm.Hobbies，只能使用原始类型。这就是你能做的：

var hobbyIds = vm.Hobbies.Select(h => h.HobbyId).ToList();
var newHobbies = db.Hobbies.Where(x => hobbyIds.Contains(x.HobbyID)).ToList();

Answer 2

要避免该异常，您可以先从vm.Hobbies集合中选择ID，然后再过滤您需要的爱好：

var Ids=vm.Hobbies.Select(x=>x.HobbyID);

var newHobbies = db.Hobbies.Where(x => Ids.Contains(x.HobbyID)).ToList();
// another option could be: db.Hobbies.Where(x => Ids.Any(id=>id==x.HobbyID)).ToList();    
originalPerson.Hobbies = newHobbies;

Answer 3

The overall construction would be like such:

Your view takes the view model which displays the list of hobbies
You have an action to add a new Hobby passing a Hobby entity back as part of the action
You simply make a call back to the database to create the new association between the Person and Hobby
Add the Hobby entity to the collection in Person
Return passing the view model back to the view

In general EF and MVC are much easier to work with when you try to interact with your entities as whole entities and don't try to micromanage yourself things such as IDs, entity hydration, and so on.

Answer 4

You could join by ID instead of fetching data from the DB:

originalPerson.Hobbies = db.Hobbies.Join(vm.Hobbies, h => h.ID, 
    v => v.HobbyID, (h, v) => h).ToList();

How to update related data using entity framework 6

4 个答案: