I'm using EF4 and new to it. I have a many to many in my project and cannot seem to work out how to insert or update. I have build a small project just to see how it should be coded.
Suppose I have 3 tables
After adding all the relationship and updated the model via model browser I have noticed that StudentClass does not appear, this seems to be default behaviour.
Now I need to do both an Insert and Update. How do you do it? Any code samples or link where I can download an example, or can you spare 5 mins?
In terms of entities (or objects) you have a Class
object which has a collection of Students
and a Student
object that has a collection of Classes
. Since your StudentClass
table only contains the Ids and no extra information, EF does not generate an entity for the joining table. That is the correct behaviour and that's what you expect.
Now, when doing inserts or updates, try to think in terms of objects. E.g. if you want to insert a class with two students, create the Class
object, the Student
objects, add the students to the class Students
collection add the Class
object to the context and call SaveChanges
:
using (var context = new YourContext()) { var mathClass = new Class { Name = "Math" }; mathClass.Students.Add(new Student { Name = "Alice" }); mathClass.Students.Add(new Student { Name = "Bob" }); context.AddToClasses(mathClass); context.SaveChanges(); }
This will create an entry in the Class
table, two entries in the Student
table and two entries in the StudentClass
table linking them together.
You basically do the same for updates. Just fetch the data, modify the graph by adding and removing objects from collections, call SaveChanges
. Check this similar question for details.
Edit:
According to your comment, you need to insert a new Class
and add two existing Students
to it:
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(); }
Since both students are already in the database, they won't be inserted, but since they are now in the Students
collection of the Class
, two entries will be inserted into the StudentClass
table.
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