EF Code First: Join table is empty

I have two Entity Framework Code First POCOs. I can insert new Site and Technology items into the database easily, but I can't seem to associate Sites with Technologies. When I do this ...

if(!site.Technologies.Any( o => (o.Name == technology.Name &&  technology.Source == source)))
{
  site.Technologies.Add(technology);
  technology.Sites.Add(site);
}

I get a duplicate key error. If I remove the second line from the conditional, the join table TechnologiesSites does not get any records! How do I do an insert in EF Code First?

public class Site
{
开发者_运维问答    public Site()
    {
        Technologies = new HashSet<Technology>();
    }

    [Key]
    public string Hostname { get; set; }
    public virtual ICollection<Technology> Technologies { get; set; }
}

and

public class Technology
{
    [Key, Column(Order = 0)]
    public string Name { get; set; }
    [Key, Column(Order = 1)]
    public string Source { get; set; }
    public virtual ICollection<Site> Sites { get; set; }

    public Technology()
    {
        Sites = new HashSet<Site>();
    }
}

You have to check both directions:

if(!site.Technologies.Any( o => (o.Name == technology.Name &&  technology.Source == source)) &&
!technology.Sites.Any(x => x.Hostname == site.Hostname)
)
{
  site.Technologies.Add(technology);
  technology.Sites.Add(site);
}