tagをcreateしたときに中間テーブルgroup_tagsにtag_idを自動登録したい。JSON形式・nativeJavaScriptを使用しています。

  has_many:group_tags
  has_many:tags,through: :group_tags

  belongs_to :group
  belongs_to :tag

  has_many:group_tags
  has_many:groups,through: :group_tags

  def create
    Tag.create(tag_params)
  end

  private
  def tag_params
    params.permit(:title,group_ids:[])
  end

{"authenticity_token"=>"Bm7Oa...", "title"=>"aaaaa", "commit"=>"作成", "group_ids"=>["1"], "controller"=>"tags", "action"=>"create"}

{"title"=>"aaaaa", "group_ids"=>["1"]}

  def create
    @tag=Tag.create(tag_params)
    respond_to do |format|
      format.html {redirect_to :root}
      format.json {render json: @tag}
    end
  end

  private
  def tag_params
    params.permit(:title,group_ids:[])
  end

form_tag_create.addEventListener('submit',function(e){

  e.preventDefault();

  let method=this.getAttribute("method");
  let url=this.getAttribute("action");
  let formData=new FormData(this);
  let jsonData={};

  formData.forEach(function(value, key){
    if(key.endsWith("_ids[]")){
      jsonData[key]=[value];
    }else{
      jsonData[key]=value;
    }
  });

  let json=JSON.stringify(jsonData);

  let xhr=new XMLHttpRequest();
  xhr.open(method,`${url}.json`);
  xhr.setRequestHeader('Content-Type', 'application/json');
  xhr.send(json);

});

{"authenticity_token"=>"Bm7Oa...", "title"=>"bbbbb", "group_ids[]"=>["1"], "controller"=>"tags", "action"=>"create", "format"=>"json", "tag"=>{"title"=>"bbbbb"}}

{"title"=>"bbbbb"}