PATCH in web API for Many to Many relationships using EF - asp.net-core

Question

I'm trying to remove a tag from a list of tags using a PATCH request. But while saving changes to the db context I get the following error:.
This the the PATCH code.
[HttpPatch("{id:int}/{field}", Name = "UpdateNote")]
[ProducesResponseType(StatusCodes.Status204NoContent)]
[ProducesResponseType(StatusCodes.Status404NotFound)]
public IActionResult UpdateNote(int id, string field, JsonPatchDocument<CreateNoteDTO> patchNoteDTO)
{
if (id == 0 || patchNoteDTO == null)
return BadRequest();
var note = _db.Notes.AsNoTracking().FirstOrDefault(u => u.ID == id);
if (note == null)
return NotFound();
CreateNoteDTO createNoteDTO = _mapper.Map<CreateNoteDTO>(note);
if (field.Equals("tags"))
{
var tags = _db.Tags.Where(u => u.Notes.Any(x => x.ID == note.ID)).AsNoTracking().ToList();
List<TagDTO> tagDTOs = new List<TagDTO>();
if (tags.Count > 0)
{
foreach (var tag in tags)
{
TagDTO tagDTO = _mapper.Map<TagDTO>(tag);
tagDTOs.Add(tagDTO);
}
}
createNoteDTO.Tags = tagDTOs;
}
patchNoteDTO.ApplyTo(createNoteDTO, ModelState);
if (!ModelState.IsValid)
return BadRequest(ModelState);
Note updateNote = _mapper.Map<Note>(createNoteDTO);
updateNote.ID = id;
updateNote.ChangedDate = DateTime.Now;
_db.Update(updateNote);
if (!field.ToLower().Equals("category"))
_db.Entry(updateNote).Property(u => u.FKCategory).IsModified = false;
_db.Entry(updateNote).Property(u => u.CreatedDate).IsModified = false;
_db.SaveChanges();
return NoContent();
}
My ApplicationDBContext looks like this:
public class ApplicationDbContext : DbContext
{
public ApplicationDbContext(DbContextOptions<ApplicationDbContext> options) : base(options)
{
}
public DbSet<Note> Notes { get; set; }
public DbSet<Category> Categories { get; set; }
public DbSet<Tag> Tags { get; set; }
protected override void OnModelCreating(ModelBuilder modelBuilder)
{
//Write Fluent API configurations here
modelBuilder.Entity<Note>()
.HasOne(e => e.Category)
.WithMany(e => e.Notes)
.HasForeignKey(e => e.FKCategory)
.OnDelete(DeleteBehavior.Cascade);
modelBuilder.Entity<Note>()
.HasMany(n => n.Tags)
.WithMany(t => t.Notes)
.UsingEntity(j => j
.ToTable("NoteTag")
.HasData
(
new { NotesID = 1, TagsTagID = 1 },
new { NotesID = 2, TagsTagID = 2 }
));
modelBuilder.Entity<Category>().HasData
(
new Category
{
CategoryID = 1,
CategoryName = "testCategory1",
CreatedDate = DateTime.Now
},
new Category
{
CategoryID = 2,
CategoryName = "testCategory2",
CreatedDate = DateTime.Now
}
);
modelBuilder.Entity<Tag>().HasData
(
new Tag
{
TagID = 1,
TagName = "testTag1",
CreatedDate = DateTime.Now
},
new Tag
{
TagID = 2,
TagName = "testTag2",
CreatedDate = DateTime.Now
}
);
modelBuilder.Entity<Note>().HasData
(
new Note
{
ID = 1,
Title = "testTitle1",
Description = "testDescription1",
FKCategory = 1,
Deleted = false,
CreatedDate = DateTime.Now,
ChangedDate = DateTime.Now,
},
new Note
{
ID = 2,
Title = "testTitle2",
Description = "testDescription2",
FKCategory = 2,
Deleted = false,
CreatedDate = DateTime.Now,
ChangedDate = DateTime.Now,
}
);
}
}
Any suggestions?
Thanks in advance.
Initially I tried without the following if condition:
CreateNoteDTO createNoteDTO = _mapper.Map<CreateNoteDTO>(note);
if (field.Equals("tags"))
{
var tags = _db.Tags.Where(u => u.Notes.Any(x => x.ID == note.ID)).AsNoTracking().ToList();
List<TagDTO> tagDTOs = new List<TagDTO>();
if (tags.Count > 0)
{
foreach (var tag in tags)
{
TagDTO tagDTO = _mapper.Map<TagDTO>(tag);
tagDTOs.Add(tagDTO);
}
}
createNoteDTO.Tags = tagDTOs;
}
But I got a different error:
I wanted to remove a tag form the following object using a PATCH request:
Note Object