ruby on rails - how to remove records from the joined table? -

please solve problem.

table posts

create_table "posts", force: :cascade |t|   t.string   "title"   t.text     "body" end 

table tags

create_table "tags", force: :cascade |t|   t.string   "tagname" end 

joined table

create_table "posts_tags", id: false, force: :cascade |t|   t.integer "post_id"   t.integer "tag_id" end 

model post

class post < activerecord::base   has_and_belongs_to_many :tags end 

model tag

class tag < activerecord::base   has_and_belongs_to_many :posts end 

i need remove of association table posts_tags.

i form set id tags:

[3, 18, 21] 

here controller handles set id_tags:

  def update     if @post.update_attributes(post_params)       #add_new_tags(@post)       p '------------------1'       p params['delete_tags']       p '------------------2'       destroy_tags(params['delete_tags'], @post)       flash[:success] = t :post_updated       redirect_to user_post_path(@user, @post)     else       flash[:error] = t :post_not_updated       render  'edit'     end   end    private      def destroy_tags(tags,post)       tags.each |tag|           p '=================='           p tag           tag_del = post.tags.find_by_post_id(:post_id => post.id)           if teg_del             post.tags.delete(tag_del)           end       end     end          def post_params       params.require(:post).permit(:delete_tags)     end   

as result, following error message:

undefined method `find_by_post' #<tag::activerecord_associations_collectionproxy:0x007f097ed78ac8>     

the console displays following message:

processing postscontroller#update html   parameters: {"utf8"=>"✓", "authenticity_token"=>"jfnofjvn2ddqiwidk/swerfkipfj9btydwjy4qfbfl8ghro7tnzhbq4mcw+udydn9atleamfcpdlmhbvhmsdhq==", "post"=>{"title"=>"corrupti.ggh", "body"=>"suscipit ut odit labore fugiat quia aliquam."}, "tagnames"=>"", "delete_tags"=>["3", "18", "21"], "commit"=>"Сохранить post", "locale"=>"ru", "user_id"=>"24", "id"=>"359"}   post load (0.2ms)  select  "posts".* "posts" "posts"."id" = ? limit 1  [["id", 359]]   user load (0.1ms)  select  "users".* "users" "users"."id" = ? limit 1  [["id", 24]]   user load (0.1ms)  select  "users".* "users" "users"."remember_token" = ? limit 1  [["remember_token", "15166203712e74cc4638f34991c141f85c04a0e0"]]   cache (0.0ms)  select  "users".* "users" "users"."id" = ? limit 1  [["id", "24"]]    (0.1ms)  begin transaction    (0.0ms)  commit transaction "------------------1" ["3", "18", "21"] "------------------2" "==================" "3" completed 500 internal server error in 6ms (activerecord: 0.5ms) 

since have join table, tags table doesn't have post_id, find_by won't work. isn't needed though because post.tags retrieve tags connected post anyway.

you lot of functionality handling habtm relationships , forms out of box rails using things collection_check_boxes don't think need destroy tags method unless i'm misunderstanding trying accomplish. might consider adding/removing tag associations posts this:

post - has_and_belongs_to_many :tags
tag - has_and_belongs_to_many :posts

post update form:

<legend>tags</legend> <div class="form-group">   <%= f.collection_check_boxes(:tag_ids, tag.all, :id, :name) %> </div> 

make sure allow :tag_ids => [] in controller's post_params

the allow update tags belonging post adding new tags checked , removing tags unchecked in form , can have normal create , update controller actions without need destroy_tags method.