Where to put methods (in entity or repo)

I have two EF generated classes.

public partial class admin
{
    public admin()
    {
        this.user = new HashSet<user>();
    }
    public int id { get; set; }//PK
    //Other properties omitted for brevity
    public virtual ICollection<user> user { get; set; }
}

And

public partial class user
{  
    public string username { get; set; }//PK
    public string passwd { get; set; }
    public int admin_id { get; set; }//FK
    //Other properties omitted for brevity
    public virtual admin admin { get; set; }
}

users belong to an admin by using FK admin_id. If admin_id and username are equal then the user is an admin. Example: users 'Messi', 'Neymar', 'Suarez' & '123' all have admin_id 123. So user '123' is an admin. (Maybe not the best approach but it is not relevant for the question.)

Since these two classes are EF auto-generated and can change in the future I have another partial user class with the same namespace but in a different folder(so the methods keep preserved):

public partial class user
{
    public bool isAdmin()
    {
        return admin_id.ToString().Equals(username);
    }
}

I also have this user repository:

public class EFUserRepo : IUserRepo
{
    private Entities context = new Entities();
    public IQueryable<user> Users { get { return context.user; } }
    public user getUserByPK(string username)
    { 
        return context.user.Find(username);
    }
    public user deleteUser(string username){ return null; }//Yet to implement
    public bool saveUser(user user){ return false; }//Yet to implement
}

And I want to make another method for obtaining the useradmin of a given user, like this:

public user getUserAdmin(string username){ }//Note the return type is user, not admin!

My question is, where do I put this method?

I can put it in the EFUserRepo like this:

public user getUserAdmin(string username)
{
    user user = getUserByPK(username);
    if (user == null) return null;
    return context.user.Find(user.admin_id);            
}

public user getUserAdmin(user user)//Not relevant for question, but might be insightful for others
{
    return getUserAdmin(user.username);
}

And in my controller call this:

user adminUser = repo.getUserAdmin(loggedOnUser.username);//or use repo.getUserAdmin(loggedOnUser) for same result.

Or I can put it in the partial user class like this:

public user getUserAdmin()
{
    return this.admin.user.Where(x => x.isAdmin()).FirstOrDefault();
    //Due to DB setup always returns 1 user.
}

And in my controller call this:

user adminUser = loggedOnUser.getUserAdmin();

I absolutely have no idea what's the best approach.

What if I also want to make a method like:

public admin getAdmin(string username){ }//Note the return type is admin here, not user

Then I can add this to my user repository:

public admin getAdmin(string username)
{
    user user = getUserByPK(username);
    if (user == null) return null;
    return user.admin;
    //return context.admin.Find(user.admin_id);//Also works, but is it best practise to access the admin collection from within the userrepo, think not
    //return new EFAdminRepo().getAdminByPK(user.admin_id)//Also works, but seams really ugly
}

public admin getAdmin(user user)//Again, not relevant for question, but might be insightful for others
{
    return getAdmin(user.username);
}

And in my controller call this:

admin admin = repo.getAdmin(loggedOnUser.username);//or use repo.getAdmin(loggedOnUser) for same result.

Or I can put it in the partial user class like this:

public admin getAdmin()
{
    return this.admin.user.Where(x => x.isAdmin()).First().admin;
}

And in my controller call this:

admin admin = loggedOnUser.getAdmin();

Real question might be, do I use

ObjectX obj = repo.obtainMethodForObjectX(entity.params);

or

ObjectX obj = entity.obtainMethodForObjectX();