使用相同的上下文实例进行整个操作，您的生活将更容易： p>

  using（var ctx = new MyContext（））
 {
 Article article = ctx.Articles.Single a => a.Id == articleId）; 
标签标签= ctx.Tags.SingleOrDefault（t => t.UrlSlug == tagUrl）; 
 if（tag == null）
 {
 tag = new Tag（）{...} 
 ctx.Tags.AddObject（tag）; 
} 

 article.Tags.Add（tag）; 
 ctx.SaveChanges（）; 
} 
 

如果您不想从数据库加载文章（该查询是多余的，如果你知道那篇文章存在）你可以使用：

  using（var ctx = new MyContext（））
 {
 Article article = new Article（）{Id = articleId}; 
 ctx.Articles.Attach（article）; 

标签标签= ctx.Tags.SingleOrDefalut（t => t.UrlSlug == tagUrl）; 
 if（tag == null）
 {
 tag = new Tag（）{...} 
 ctx.Tags.AddObject（tag）; 
} 

 article.Tags.Add（tag）; 
 ctx.SaveChanges（）; 
} 
 

I am getting a primary key violation error when I attempt to add an item with a many-to-many relationship:

I have two classes - Articles and Tags which have a many-to-many relationship :

public class Article
{
    public int ID { get; set; }
    public string Text { get; set; }
    public   ICollection<Tag>  Tags { get; set; }
}

public class Tag
{ 
    [Key]
    public string UrlSlug { get; set; }
    public string Name { get; set; }
    public ICollection<Article> Articles{ get; set; }
}

When I add a new Article I allow the user to input any Tags and then I want to create a new Tag if the Tag isn't created yet in the database or add the Tag to the Tags collection of the Article object if the Tag already exists.

Therefore when I am creating the new Article object I call the below function:

public static Tag GetOrLoadTag(String tagStr)
        {
            string tagUrl = Tag.CreateTagUrl(tagStr);
            var db = new SnippetContext();
            var tagFromDb = from tagdummy in db.Tags.Include(x => x.Articles)
                            where tagdummy.UrlSlug == tagUrl
                            select tagdummy;
            if (tagFromDb.FirstOrDefault() != null)
            { return tagFromDb.FirstOrDefault(); }
            else
            {
                //create and send back a new Tag
            }

        }

This function basically checks if there is an available Tag in the database and if so returns that Tag which is then added to the Tag collection of the Article object using article.Tags.Add().

However, when I attempt to save this using the below code I get a Violation of PRIMARY KEY constraint error

 db.Entry(article).State = EntityState.Modified;
 db.SaveChanges();

I can't figure out how I should go about just creating a relationship between the Article and the already existing Tag.

Use the same context instance for the whole processing of your operation and your life will be much easier:

using (var ctx = new MyContext())
{
    Article article = ctx.Articles.Single(a => a.Id == articleId);
    Tag tag = ctx.Tags.SingleOrDefault(t => t.UrlSlug == tagUrl);
    if (tag == null) 
    {
       tag = new Tag() { ... }
       ctx.Tags.AddObject(tag);
    }

    article.Tags.Add(tag);
    ctx.SaveChanges();
}

If you don't want to load the article from database (that query is redundant if you know that article exists) you can use:

using (var ctx = new MyContext())
{
    Article article = new Article() { Id = articleId };
    ctx.Articles.Attach(article);

    Tag tag = ctx.Tags.SingleOrDefalut(t => t.UrlSlug == tagUrl);
    if (tag == null) 
    {
       tag = new Tag() { ... }
       ctx.Tags.AddObject(tag);
    }

    article.Tags.Add(tag);
    ctx.SaveChanges();
}