Rails: Form doesn't add data to model

I have a form for creating a new :thing, with a collection_select field to enter an existing :thing the new :thing is related to. Each :thing has_many :things, through an intermediary model :related_things, which has a thing_a_id and thing_b_id. So when I fill in the field and click submit, a :related_thing is supposed to be created with thing_a_id and thing_b_id equal to the two thing_ids, respectively. But no such :related_thing is created; the form doesn't do anything. The other textfields do work though. What's wrong with my code?

I'm using Rails 4.0.10.

Things/new View:

<h1>Add Something!</h1>
<p>
  <%= form_for @thing, :url => things_path, :html => { :multipart => true } do |f| %>

    <%= f.text_field :name, :placeholder => "Name of the thing" %>
    <br>
    <%= f.label :related_things %>
    <%= f.collection_select :related_things, Thing.all, :id, :name %>
    <br>
    <%= f.label :display_picture %>
    <%= f.file_field :avatar %>
    <br>
    <%= f.submit "Submit", class: "btn btn-primary" %>
  <% end %>
</p>

Thing Model:

class Thing < ActiveRecord::Base
  has_many :related_things
  has_many :things, :through => :related_things
  has_attached_file :avatar, :styles => { :medium => "300x300>", :thumb => "30x30!" }, :default_url => "/images/:style/missing.png"
  validates_attachment_content_type :avatar, :content_type => /\Aimage\/.*\Z/

  def related_things
    related_thing_ids = RelatedThing.
        where("thing_a_id = ? OR thing_b_id = ?", self.id, self.id).
        map { |r| [r.thing_a_id, r.thing_b_id] }.
        flatten - [self.id]
    Thing.where(id: related_thing_ids)
  end

  def related_thing_ids=(ids)
    ids.each do |id|
      record = RelatedThing.where(thing_a_id: self.id, thing_b_id: id).first
      record ||= RelatedThing.where(thing_a_id: id, thing_b_id: self.id).first
      record ||= RelatedThing.create!(thing_a_id: self.id, thing_b_id: id)
    end
  end

end

RelatedThing Model:

class RelatedThing < ActiveRecord::Base
  has_many :things
end

Things Controller:

class ThingsController < ApplicationController

  def show
    @thing = Thing.find(params[:id])
    @related_thing = RelatedThing.all
    @thing.things.build  
  end

  def new
    @thing = Thing.new
    @things = Thing.all
  end

  def create
    @thing = Thing.new(thing_params)
    if @thing.save
      redirect_to @thing
    else
      render 'new'
    end
  end

  private

    def thing_params
      params.require(:thing).permit(:name, :image_path, :avatar)
    end

end

RelatedThings Controller:

class RelatedThingsController < ApplicationController
  def new
    @things = Thing.all.by_name
  end

  def create
    @things = Thing.all.by_name
  end

  def edit
    @things = Thing.all.by_name
  end
end