Rails association don't work with attributes - ruby-on-rails

Question

I have a problem with my association in ruby on rails.
I have the association :
class Play < ApplicationRecord
has_many :program_plays
has_many :programs, through: :program_plays
end
class ProgramPlay < ApplicationRecord
belongs_to :program
belongs_to :play
end
class Program < ApplicationRecord
has_many :program_plays
has_many :plays, through: :program_plays
accepts_nested_attributes_for :plays
end
When i create a Program i want to associate one or several Play. The Play are create beforehand. The Play have got many attributes whose :title and :id of course.
The program_controller :
class Admin::ProgramsController < Admin::AdminController
def index
#programs = Program.all
end
def new
#program = Program.new
#program.plays.build
end
def create
#program = Program.new(program_params)
if #program.save
redirect_to admin_programs_path, notice: ''
else
end
end
private
def set_program
#program = Program.find(params[:id])
end
def program_params
params.require(:program).permit(:start_date, plays_attributes: [:title])
end
end
The form of Program (with the simple_form gem) :
<%= simple_form_for [:admin, #program] do |f| %>
<%= f.error_notification %>
<%= f.association :plays %>
<%= f.input :start_date, as: :datetime, minute_step: 15, label: 'Heure' %>
<%= f.button :submit, 'Poster', class: 'button red-full' %>
<% end %>
Now when i want to display the Program in my index, i also want to display the title of associated Play. But when i write :
program.plays.title
an error tell me that title is not a method of program... Why ? I don't understand. The association is real because when i puts in the rails console :
program.plays.to_a
i have :
Play Load (5.4ms) SELECT "plays".* FROM "plays" INNER JOIN "program_plays" ON "plays"."id" = "program_plays"."play_id" WHERE "program_plays"."program_id" = $1 [["program_id", 21]]
=> []
Can you see the problem ? I look for it for a longtime ago... I am desperate...
UPDATE :
In the rails console when i do program.plays.first.title :
Play Load (0.5ms) SELECT "plays".* FROM "plays" INNER JOIN "program_plays" ON "plays"."id" = "program_plays"."play_id" WHERE "program_plays"."program_id" = $1 ORDER BY "plays"."id" ASC LIMIT $2 [["program_id", 22], ["LIMIT", 1]]
NoMethodError: undefined method `title' for nil:NilClass
When i do :
program.plays.each do |play|
play.title
end
The result is :
Play Load (0.5ms) SELECT "plays".* FROM "plays" INNER JOIN "program_plays" ON "plays"."id" = "program_plays"."play_id" WHERE "program_plays"."program_id" = $1 [["program_id", 22]]
=> []
Here we can see that title is empty...
UPDATE 2 :
create_table "plays", force: :cascade do |t|
t.string "title"
t.text "body"
t.datetime "created_at", null: false
t.datetime "updated_at", null: false
end
create_table "program_plays", force: :cascade do |t|
t.bigint "program_id"
t.bigint "play_id"
t.index ["play_id"], name: "index_program_plays_on_play_id"
t.index ["program_id"], name: "index_program_plays_on_program_id"
end
create_table "programs", force: :cascade do |t|
t.datetime "start_date"
end
Maybe the problem is here...
UPDATE 3 :
> program = Program.new
=> #<Program:0x00000000068ce8e8 id: nil, start_date: nil>
[15] pry(main)> program.id = 1
=> 1
> program.plays.build(Play.find_by(id: 10))
Play Load (6.2ms) SELECT "plays".* FROM "plays" WHERE "plays"."id" = $1 LIMIT $2 [["id", 10], ["LIMIT", 1]]
ArgumentError: When assigning attributes, you must pass a hash as an argument.
from /home/coeurcoeur/.rvm/gems/ruby-2.5.0/gems/activemodel-5.1.4/lib/active_model/attribute_assignment.rb:28:in `assign_attributes'
> program.save
(0.1ms) BEGIN
(0.3ms) ROLLBACK
=> false

Answer 1

The associations looks good. the problem that you have is that you are calling program.plays.title but have in mind that program.plays will have a collection of plays and not just one so you can't call directly the title there.
it depends on what you are trying to do, for example if you want the title of the first play, you can do something like program.plays.first.title
or if you want to print all the plays title you can do something like
program.plays.each do |play|
play.title
end
and I am assuming program is filled with an existing program in your code
Another problem that you have is that you are not permitting the program_id on the controller so when you create the plays, the attribute program_id that has the association isn't saved because isn't permitted, so just add program_id on your params method like this
def program_params
params.require(:program).permit(:start_date, plays_attributes: [:title, :program_id])
end