Entity Framework Code First Existing Database set EntityState.Unchanged on Many-to-Many

Question

I have an existing database I'm coding against with Entity Framework 6 Code First. I have a many-to-many relationship that is working for Selects, Inserts and Deletes. I'm having an issue with EF adding an extra insert to the many-to-many table for an existing relationship.

Schema:

DbContext:

    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
       Database.SetInitializer<MainContext>(null);

       modelBuilder.Entity<DocumentType>()
                .HasMany(u => u.DocumentStatuses)
                .WithMany()
                .Map(m =>
                {
                    m.MapLeftKey("DOCUMENT_TYPE_ID");
                    m.MapRightKey("DOCUMENT_STATUS_ID");
                    m.ToTable("DOCUMENT_TYPES_DOCUMENT_STATUSES");
                });

       base.OnModelCreating(modelBuilder);
    } 

DTOs:

    [Table("DOCUMENT_TYPES")]
    public class DocumentType
    {
        [Key]
        [Column("DOCUMENT_TYPE_ID")]
        public int? Id { get; set; }

        [Required]
        [Column("TYPE_NAME")]
        public string TypeName { get; set; }

        [Required]
        [Column("IS_ACTIVE")]
        public bool IsActive { get; set; }

        [Display(Name = "Updated By")]
        [Column("LAST_UPDATED_BY")]
        public string LastUpdatedBy { get; set; }

        [Display(Name = "Updated Date")]
        [Column("LAST_UPDATED_DATE")]
        public DateTimeOffset? LastUpdatedDate { get; set; }

        public virtual List<DocumentStatus> DocumentStatuses { get; set; } 

        public DocumentType()
        {
            DocumentStatuses = new List<DocumentStatus>();
        }
    }

[Table("DOCUMENT_STATUSES")]
public class DocumentStatus
{
    [Key]
    [Column("DOCUMENT_STATUS_ID")]
    public int? Id { get; set; }

    [Required]
    [Column("STATUS_NAME")]
    public string StatusName { get; set; }

    [Required]
    [Column("IS_COMPLETE")]
    public bool IsComplete { get; set; }

    [Required]
    [Column("IS_ACTIVE")]
    public bool IsActive { get; set; }

    [Display(Name = "Updated By")]
    [Column("LAST_UPDATED_BY")]
    public string LastUpdatedBy { get; set; }

    [Display(Name = "Updated Date")]
    [Column("LAST_UPDATED_DATE")]
    public DateTimeOffset? LastUpdatedDate { get; set; }
}

Repository Update:

    public bool Update(DocumentType entity, string updatedBy)
            {
                DateTimeOffset updatedDateTime = DateTimeOffset.Now;

                entity.LastUpdatedBy = updatedBy;
                entity.LastUpdatedDate = updatedDateTime;

                using (var db = new MainContext())
                {
                    db.DocumentTypes.Add((DocumentType)entity);
                    db.Entry(entity).State = EntityState.Modified;

                    foreach (var item in entity.DocumentStatuses)
                    {
                        if (item.Id != null)
                            db.Entry(item).State = EntityState.Unchanged;
                    }

                    db.SaveChanges();
                }

                return true;
            }

The loop that contains:

    if (item.Id != null)
     db.Entry(item).State = EntityState.Unchanged;

Prevents new records from being added, but EF still tries to insert into DOCUMENT_TYPES_DOCUMENT_STATUSES a duplicate of the existing many-to-many records.

Unit Test:

        [TestMethod()]
        public void UpdateTest()
        {
            DocumentTypeRepository documentTypeRepository = new DocumentTypeRepository();
            DocumentType type = NewDocumentType(true);
            DocumentType typefromDb;
            string updatedBy = "DocumentTypeRepositoryTests.UpdateTest";
            bool actual;

            type.IsActive = true;
            type.TypeName = RandomValues.RandomString(18);
            type.DocumentStatuses.Add(DocumentStatusRepositoryTests.NewDocumentStatus(true));

            actual = documentTypeRepository.Update(type, updatedBy);
            Assert.AreEqual(true, actual);

            typefromDb = documentTypeRepository.GetById((int)type.Id);
            Assert.AreEqual(type.DocumentStatuses.Count, typefromDb.DocumentStatuses.Count);
        }

How can I set the many-to-many table to EntityState.Unchanged when it already exists?

Answer 1

Try replacing

db.DocumentTypes.Add((DocumentType)entity);
db.Entry(entity).State = EntityState.Modified;

with

db.Entry(entity).State = entity.Id == null 
    ? EntityState.Added
    : EntityState.Modified;

When doing CRUD operations with detached entities, it's easier not to use the DbSet operations and only manipulate entity state entries. At least, it leads to less errors.

Answer 2

Two things are important to note here:

1. When you Add an entity to a context, the whole object graph owned by the entity is marked as Added .

2. In any association this also marks the associations as new. However, while in a 1:n or 1:1 association the state of the association changes when the state of one of the ends changes, in many-to-many associations there is a part of the association that is hidden and that will always remain Added . (Under the hood a many-to-many association is a 1:n:1 association, the n part is not visible in your code).

So this statement...

db.DocumentTypes.Add(entity);

causes the associations to get the Added state, and the subsequent...

db.Entry(entity).State = EntityState.Modified;

doesn't change this any more.

You have to make sure that for an existing DocumentType the state is never changed to Added , which you can do by checking its Id values, same as you do for DocumentStatus .

Also, you have to change the way you set the state of DocumentStatuses :

foreach (var item in entity.DocumentStatuses)
{
    db.Entry(item).State = item.Id != null
                               ? EntityState.Unchanged
                               : EntityState.Added;
}