Entity Framework - Can't Delete and Insert to DbContext in the same method

Question

So the 3 tables I'm using are Event and Tag tables that have a many-to-many relationship, which is 'simulated' via a bridging table that has a one-to-many relationship with each table, EventTag , whose foreign keys make up a composite primary key.

In my code, I guess albeit very inefficient, I'm trying to delete entries from the EventTag table when adding Tag s to be associated with a particular Event or changing which existing tags are now associated with it.

The code below works fine when I only delete tags associated with an event or when I only add new tags, however when I add and delete tags in the same call, the tags only get deleted, and no changes are made to add the new ones.

I tried taking out the first call to context.SaveChanges(); putting it all in one (using DbContext ... ) block but neither seemed to work. I've no idea what the problem could be.

public async Task<string> Post(AddTagsRequest request)
    {
        string problem = "";
        try
        {
            using (DbContext context = new DbContext())
            {
                var eventtagstodelete =
                    await context.EventTags.Where(eventtag => eventtag.EventId == request.EventId).ToListAsync();
                foreach (var eventtagtodelete in eventtagstodelete)
                {
                    context.Entry(eventtagtodelete).State = EntityState.Deleted;
                }

                List<Tag> Tags = request.Tags.Select(tag => new Tag {Value = tag}).ToList();

                List<string> tagIds = new List<string>();

                foreach (var tag in Tags)
                {
                    List<Tag> foundTags = await context.Tags.Where(row => row.Value == tag.Value).ToListAsync();
                    if (foundTags.Count > 0)
                    {
                        tagIds.Add(foundTags.First().Id);
                    }
                    else
                    {
                        context.Tags.Add(new Tag
                        {
                            Value = tag.Value
                        });
                    }
                }

                foreach (var tagId in tagIds)
                {
                    context.EventTags.Add(new EventTag
                    {
                        TagId = tagId,
                        EventId = request.EventId
                    });
                }
                await context.SaveChangesAsync();
                return "success";
            }
        }
        catch (Exception e)
        {
            while(e.InnerException != null){
                e = e.InnerException;
            }
            return "failure - Message: " + e.Message;
        }
    }

public class AddTagsRequest
{
    public List<string> Tags { get; set; }
    public string EventId { get; set; }
}

---

public class Event : EntityData
{
    public string Name { get; set; }
}

---

public class Tag : EntityData
{
    public string Value { get; set; }
}

---

public class EventTag : EntityData
{
    public Event Event { get; set; }
    public string EventId { get; set; }
    public Tag Tag { get; set; }
    public string TagId { get; set; } 
}

---

    public EventTagConfig()
    {
        HasKey(eventTag => new { eventTag.EventId, eventTag.TagId });
    }

Update 1

Added Event , Tag , and EventTag classes, as well as the configuration class EventTagConfig to configure the EventTag table.

Changed code from 2 using and 2 context.SaveChanges(); to only 1 of each.

When I run it like this I get the following error:

Modifying a column with the 'Identity' pattern is not supported. Column: 'Id'. Table: 'CodeFirstDatabaseSchema.EventTag'.

Answer 1

There wasn't a problem with having multiple saves/contexts - I just did something stupid.

I actually removed the second using, and saved any time I needed to persist changes. The problem I had was in my if-else block I added the Id of an existing tag to the tagIds list but when adding another, I wasn't then adding the new Id to the list, so it wasn't adding the EventTag entry this time, but when I run it a second time, the 'new'-now-existing tag is found, and it's Id added to the list.

To fix I add all new tags to a list - newTagList , in the else block, then call context.Tags.AddRange(newTagList) , followed by context.SaveChanges() and then pull the Ids from the newTagList to be added to the EventTag table.

To note - @UrbanEsc that error I posted in the update came about as a result of the change suggested by @Henk_Holterman, and was due to entries marked with EntityState.Deleted not actually being deleted when the insert was called, thus giving that error.

Anyway, thanks for your help guys.