How to create a many to many relationship in a view - asp.net-mvc-3

Question

Let me be more specific in my question.
I have a many to many relationship. I have an Opportunity and I have items.
In my Opportunity/Create view I want to be able to "add" as many items from the items list as I want. So when I click the save button on the create form, I would be potentially saving many items that would be associated with this record.
So the only way to do this is to include a click box for every item in my items table? How would I do something like Click this button to insert a new item from a drop down list?
Models:
public class Opportunity
{
public int Id { get; set; }
public string MyOpportunity { get; set; }
}
public class Items
{
public int Id { get; set; }
public string ItemName { get; set; }
}
public class ItemsToOpportunity
{
public int Id { get; set; }
public int OpportunityId { get; set; }
public int ItemId { get; set; }
}

Answer 1

Based on your question and comment, it sounds like you just need to provide a form with multiple checkboxes or so.
Then, when you post to the server, the shared name of the checkboxes becomes a comma delimited set of Ids to use.
<input type="checkbox" name="myNewOpportunityItems" value="1"><span>Item 1</span>
<input type="checkbox" name="myNewOpportunityItems" value="2"><span>Item 1</span>
<input type="checkbox" name="myNewOpportunityItems" value="3"><span>Item 1</span>
<input type="checkbox" name="myNewOpportunityItems" value="4"><span>Item 1</span>
Then, when the user selects the checkboxes of the items they want to associate, that gets posted as a comma delimited string:
public ActionResult MyPostActionResult(string myNewOpportunityItems)
{
// myNewOpportunityItems == "1,3,4";
}