_post.html.haml

%li
  = link_to "Remove from collection", remove_from_collection_posts_path(cid: params[:id], pid: post.id), data: { confirm: "Are you sure you want to remove this post?" }, remote: true

posts_controller.rb

def remove_from_collection
  if params[:pid].present?
    @collection = Collection.find(params[:cid])
    @post = Post.find(params[:pid])
    if @collection.posts.include? @post
      @collection.posts.delete(@post)
      @collection.save
      @collection.touch
      render :js => "post_removed();"
    end
  end
end

remove_from_collection.js.erb

function post_removed() {
  alert("DONE");
}

function not_removed() {
  alert("NOT DONE");
}


$.ajax({
  type: "GET",
  url: "/posts/remove_from_collection",
  data: { pid: <%= params[:pid] %>, cid: <%= params[:cid] %> },
})