Rails: Form doesn't add data to model - ruby-on-rails

Question

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

Answer 1

There are two problems causing this:
As Jamesuriah pointed out, your collection_select should use the related_things_ids field instead.
Despite that change, the field is actually being filtered out of the parameter map because of Rails' Strong Parameters.
Specifically, in your controller, the thing_params method should look like:
def thing_params
params.require(:thing).permit(:name, :image_path, :avatar, :related_things_ids)
end
Read up on strong parameters in the link above for more info. Hope that helps!

Answer 2

The collection select should be named related_thing_ids for your model to work, I believe.