Dynamic Connection String web api [on hold]

i want to using Dynamic Connection String in web api. i found a lot of topic but it's not working as follow.

public KETOAN2014Entities(string conn)
        : base("name="+conn)
    {
        this.Configuration.LazyLoadingEnabled = false;
    }



 public class AccountingObjectController : ApiController
{
    private string connstring; //can't modify
    private KETOAN2014Entities db = new KETOAN2014Entities(connstring);
    [EnableQueryAttribute]
    // GET api/AccountingObject
    public IEnumerable<AccountingObject> GetAccountingObject()
    {
        return db.AccountingObject;
    }
 }

however, when i have got database_name, how to set connstring=database_name??