Ruby on Rails : Tricky Many to Many relationship does not work - activerecord

Question

I am designing a "Lists" mapping feature in my application for transcoding purposes.
So that my mappins_list object points to a source_list object and a target_list object, both being of an instance values_list class.
The design looks classical, but it does not behave as expected:
<td><%= link_to #mappings_list.source_list.name, #mappings_list.source_list%>
raises a
undefined method `source_list' for #<ActiveRecord::Relation::ActiveRecord_Relation_MappingsList:0x00000004217718>
in the index view.
(note that one record was crated with correct attributes, it should be displayed there)
Here is the code:
# == Schema Information
#
# Table name: values_lists
#
# id :integer not null, primary key
# playground_id :integer
# code :string(255)
# name :string(255)
# description :text
# created_by :string(255)
# updated_by :string(255)
# owner_id :integer
# table_name :string(255)
# created_at :datetime
# updated_at :datetime
# software_id :integer
# software_name :string(255)
#
class ValuesList < ActiveRecord::Base
### scope
scope :pgnd, ->(my_pgnd) { where "playground_id=?", my_pgnd }
### before filter
before_create :set_code
### validation
validates :code, length: { maximum: 100 }
validates :name, presence: true, uniqueness: true, length: { maximum: 100 }
validates :description, length: { maximum: 1000 }
validates :created_by , presence: true
validates :updated_by, presence: true
validates :playground_id, presence: true
belongs_to :playground
validates :playground, presence: true # validates that the playground exists
belongs_to :owner, :class_name => "User", :foreign_key => "owner_id" # helps retrieving the owner name
belongs_to :software, :class_name => "Parameter", :foreign_key => "software_id" # helps retrieving the software name
has_many :values
has_many :mappings_lists
### private functions definitions
private
### before filters
def set_code
self.code = name.gsub(/[^0-9A-Za-z]/, '_').upcase
end
end
.
# == Schema Information
#
# Table name: mappings_lists
#
# id :integer not null, primary key
# playground_id :integer
# code :string(255)
# name :string(255)
# description :text
# created_by :string(255)
# updated_by :string(255)
# owner_id :integer
# source_list_id :integer
# target_list_id :integer
# created_at :datetime
# updated_at :datetime
#
class MappingsList < ActiveRecord::Base
### scope
scope :pgnd, ->(my_pgnd) { where "playground_id=?", my_pgnd }
### before filter
before_create :set_code
### validation
validates :name, presence: true, uniqueness: true, length: { maximum: 100 }
validates :description, length: { maximum: 1000 }
validates :created_by , presence: true
validates :updated_by, presence: true
validates :playground_id, presence: true
belongs_to :playground
validates :playground, presence: true # validates that the playground exists
belongs_to :owner, :class_name => "User", :foreign_key => "owner_id" # helps retrieving the owner name
belongs_to :source_list, :class_name => "ValuesList", :foreign_key => "source_list_id" # helps retrieving the source list name
belongs_to :target_list, :class_name => "ValuesList", :foreign_key => "target_list_id" # helps retrieving the target list name
has_many :mappings
### private functions definitions
private
### before filters
def set_code
self.code = "#{source_list.code}_TO_#{target_list.code}"
end
end
routes contains this:
resources :values_lists do
resources :values
resources :mappings_lists
end
The controller is here:
class MappingsListsController < ApplicationController
# Check for active session
before_action :signed_in_user
# Retrieve current current mapping
before_action :set_mappings_list, only: [:show, :edit, :update, :destroy]
# Retrieve all lists of values
before_action :set_values_lists
# GET /mappings_list
# GET /mappings_list.json
def index
#mappings_list = MappingsList.pgnd(current_playground).order("name")
end
# GET /mappings_list/1
# GET /mappings_list/1.json
def show
### Retrieved by Callback function
end
# GET /mappings_list/new
def new
#mappings_list = MappingsList.new
end
# GET /mappings_list/1/edit
def edit
### Retrieved by Callback function
end
# POST /mappings_list
# POST /mappings_list.json
def create
#mappings_list = MappingsList.new(mappings_list_params)
#mappings_list.updated_by = current_user.login
#mappings_list.created_by = current_user.login
#mappings_list.playground_id = current_user.current_playground_id
#mappings_list.owner_id = current_user.id
respond_to do |format|
if #mappings_list.save
format.html { redirect_to #mappings_list, notice: 'List of mappings was successfully created.' }
format.json { render action: 'show', status: :created, location: #mappings_list }
else
format.html { render action: 'new' }
format.json { render json: #mappings_list.errors, status: :unprocessable_entity }
end
end
end
# PATCH/PUT /mappings_list/1
# PATCH/PUT /mappings_list/1.json
def update
### Retrieved by Callback function
#mappings_list.updated_by = current_user.login
respond_to do |format|
if #mappings_list.update(mappings_list_params)
format.html { redirect_to #mappings_list, notice: 'List of mappings was successfully updated.' }
format.json { head :no_content }
else
format.html { render action: 'edit' }
format.json { render json: #mappings_list.errors, status: :unprocessable_entity }
end
end
end
# DELETE /mappings_list/1
# DELETE /mappings_list/1.json
def destroy
### Retrieved by Callback function
#mappings_list.destroy
respond_to do |format|
format.html { redirect_to mappings_lists_url }
format.json { head :no_content }
end
end
private
# Use callbacks to share common setup or constraints between actions.
# Check for active session
def signed_in_user
redirect_to signin_url, notice: "You must log in to access this page." unless signed_in?
end
def set_mappings_list
#mappings_list = MappingsList.pgnd(current_playground).find(params[:id])
end
# retrieve the list of values lists
def set_values_lists
#lists_of_values = ValuesList.all
end
# Never trust mappings from the scary internet, only allow the white list through.
def mappings_list_params
params.require(:mappings_list).permit(:name, :description, :source_list_id, :target_list_id)
end
end
Would the double reference to values_list turn my application into trouble ?
Thanks for your help,
Best regards,
Fred

Answer 1

In the class MappingsList < ActiveRecord::Base model
you have MappingList belonging to source_list.
which I believe is backwards:
class Order < ActiveRecord::Base
belongs_to :customer
end
#order = #customer.orders

Answer 2

OK, I got it !!!
In the index view, I cannot refer to one instance variable such as #mappings_list as I work on a list of instances.
The code works fine with :
<td><%= link_to mappings_list.source_list.name, mappings_list.source_list%>
Being a beginner at RoR, I hope my interpretation is right ...
Thanks for your help,
Best regards,
Fred