Mongodb remove array with id .Net

Question

I have collections student and class.

Class contains an array of students. Whenever a student is edited it will also be edited in all the classes that the student is in.

So when a student is deleted I also want it to be deleted from all the classes it is in.

Right now I am having to pass the whole student to pull from the class

var query = Query.EQ("Students._id", idOfStudent);
var update = Update<Class>.Pull(x => x.Students, studentToRemove);

Is there a better way to pull where I can just pass in the student id for that student?

EDITED

public class Student
{
   [BsonId]
   public Guid Id {get; set;}
   public string Name {get; set;}       
}

public class Class
{
   [BsonId]
   public Guid Id {get; set;}
   public string Grade {get; set;}
   public List<Student> Students {get; set;}  
}

Then my class collection would look like

{
    "_id" : NUUID("..."),
    "Grade" : "5th",
    "Students" : [
           {
              "_id" : NUUID("..."),
               "Name" : "Chris"
           },
            {
              "_id" : NUUID("..."),
               "Name" : "Bob"
           }
}

And my student collection would look like

  {
    "_id" : NUUID("..."),
    "Name" : "Chris"
  }

When someone deletes Student 'Chris' they supply an id which will be used to find the student and delete it, but I also want to take that id and pull that student from all the Classes they are in.

Can you show us some sample documents from both the class and the student collections? — chridam
– chridam, Commented Apr 17, 2015 at 10:43

mnemosyn · Accepted Answer · 2015-04-17 16:03:51Z

1

Yes, you can, using the unique identifier in your student:

var joe = new Student { Name = "Joe", Id = Guid.NewGuid() };
/// ....
var query = Query<Class>.EQ(p => p.Id, doc.Id);
var update = Update<Class>.Pull(r => r.Students, p => p.EQ(q => q.Id, joe.Id));

Hints:

Remember that GUIDs provide bad data locality - that can be a problem or it can be advantageous, but most data roughly has creation-time locality, so ObjectId is often a better bet.
I'd suggest you use the same coding style - either expression based, or string based, but your query uses the string-based notation while the update uses an expression

answered Apr 17, 2015 at 16:03

mnemosyn

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2 Comments

Beastwood Over a year ago

Thanks so much great answer. Thanks for the hints. Question though. I always do use expression based, but I only have the id of the student. So in your example above I don't have doc.Id just joe.Id. I wouldn't let me do Query<Class>.EQ(p => p.Students.Id, joe.Id), so that is why I put a string in there.

mnemosyn Over a year ago

Ah, I see. In that case, you're forced to use the string syntax indeed.

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