has_many join form with collection checkboxes not saving more than one checkbox value

I am working on a form for a editorial calendar app. I have two things going out that are pretty similar and not working.

Working with 3 models: Platforms, Posts and Calendars. They are join tables. Platform <=> Post, Post <=> Calendars

Post/new & Post/edit form:
<div class="container">
  <div class="form-field">
    <%= form_for @post do |f| %>
    <%= f.label :title %>
    <%= f.text_field :title, required: true %> <br>
    Title is required.
  </div>

  <div class="form-field">
    <%= f.label :content%>
    <%= f.text_area :content%>
  </div>

  <div class="form-field">
    <%= f.label :link %>
    <%= f.text_field :link %>
  </div>

  <div class="file-field">
    <%= f.label :picture%>
    <%= f.file_field :picture, id: :post_picture%>
  </div>

  <%= f.hidden_field :user_id %> <br>

  <div class="form-field">
    <%= f.fields_for :platform_attributes do |platform| %>
    <%= platform.label :platform, "Social Platforms"%>
    <%= platform.collection_check_boxes :platform_ids, Platform.all, :id, :name %> <br> <br>
  </div>
  <div>
    <h4> Or Create a new platform: </h4>
    <%= platform.label :platform, 'New Platform'%>
    <%= f.fields_for :platform_attributes, @post.platforms.build do |platform| %>
    <%= platform.text_field :name%> <br> <br>
    <% end %>
  </div>
  <% end %>


  <%= f.submit class: "btn btn-sm btn-info"%>

  <% end %>
</div>

My post controller is handling the checkboxes issue, and the "schedule post" issue. It will only allow me to schedule for one calendar, and it does not save the updates and add additional calendars.

Posts Controller:
class PostsController < ApplicationController
  before_action :set_post, only: [:show, :edit, :update, :schedule_post, :destroy]

def new
    @posts = current_user.posts.select {|p| p.persisted?}
    @post = current_user.posts.build
    @platforms = Platform.all
  end

  def edit
    @calendars = current_user.calendars
    @platforms = Platform.all
  end

  def create
    @post = current_user.posts.build(post_params)
    if @post.save
      redirect_to post_path(@post)
    else
      redirect_to new_post_path
    end
  end

  def update
    @post.update(post_params)
    if @post.save
      redirect_to post_path(@post), notice: 'Your post has been updated.'
    else
      redirect_to edit_post_path(@post)
    end
  end

  def schedule_post
    @calendar_post = CalendarPost.new(calendar_post_params)
    if @calendar_post.save
      binding.pry
      redirect_to post_path(@post)
    else
      render 'show'
    end
  end

  private
  def set_post
    @post = Post.find(params[:id])
  end

  def set_calendars
    @calendars = current_user.calendars
  end

  def post_params
    params.require(:post).permit(:title, :content, :link, :finalized, :picture, :user_id, :platform_attributes => [:name])
  end

  def calendar_post_params
    params.require(:calendar_posts).permit(:post_id, :calendar_id, :date, :time)
  end
end

I want the user to be able to add a post to multiple platforms and multiple calendars because of the versatility of what someone may need.

I also have my setter in my Post model.

class Post < ApplicationRecord
  has_many :calendar_posts
  has_many :calendars, through: :calendar_posts
  has_many :platform_posts
  has_many :platforms, through: :platform_posts
  belongs_to :user

  def platform_attributes=(platform_attributes)
   platform_attributes.values.each do |platform_attribute|
      platform = Platform.find_or_create_by(platform_attribute)
      self.platforms << platform
    end
  end

thoughts? why are they not saving to more than one calendar or more than one platform if they choose to have more than one?