Entity Framework code. The first existing database. EntityState.Unchanged for many-to-many.

I have an existing database that I am coding with Entity Framework 6 Code First. I have a many-to-many relationship that works on selections, inserts and deletes. I'm having a problem with EF adding an extra insert to a many-to-many table for an existing relationship.

Scheme:

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; }
}

      

        
        
        
      

    

Storage 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;
            }

      

        
        
        
      

    

A loop containing:

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

      

        
        
        
      

    

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

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 a many-to-many table in EntityState.Unchanged if it already exists?