How to get this nested form to create a new object - ruby-on-rails

Question

My Rails 5 application includes two models, Activity and Timeslot.
activity.rb
class Activity < ApplicationRecord
belongs_to :club
has_many :timeslots, :dependent => :destroy
accepts_nested_attributes_for :timeslots
validates :club_id, presence: true
validates :name, presence: true, length: { maximum: 50 }
end
timeslot.rb
class Timeslot < ApplicationRecord
belongs_to :activity
validates :time_start, presence: true
validates :time_end, presence: true
validates :day, presence: true
#validates :activity_id, presence: true (I realised this was causing one of the errors I had)
default_scope -> { order(day: :asc) }
end
When I create my activity, I'd also like to create it's first timeslot on the same page, same form.
new.html.erb
<%= form_for(#activity) do |f| %>
<%= render 'shared/error_messages', object: f.object %>
<div class="field">
<%= f.label :name, "Class name" %>*
<%= f.text_field :name, class: 'form-control' %>
<%= f.fields_for :timeslots do |timeslots_form| %>
<%= timeslots_form.label :time_start, "Start time" %>
<%= timeslots_form.time_select :time_start %>
<%= timeslots_form.label :time_end, "End time" %>
<%= timeslots_form.time_select :time_end %>
<%= timeslots_form.label :day %>
<%= timeslots_form.select :day, (0..6).map {|d| [Date::DAYNAMES[d], d]} %>
<% end %>
</div>
<%= f.submit "Create class", class: "btn btn-primary" %>
<% end %>
My edit/update version of this seems to be working fine.
activities_controller.rb
class ActivitiesController < ApplicationController
...
def new
#activity = Activity.new
#activity.timeslots.build
end
def create
#activity = current_club.activities.build(activity_params)
##activity.timeslots.first.activity_id = #activity.id (I thought this might solve the problem, but didn't)
if #activity.save
flash[:success] = "New class created!"
redirect_to activities_path
else
render 'new'
end
end
def edit
#activity = current_club.activities.find_by(id: params[:id])
#activity.timeslots.build
end
def update
#activity = current_club.activities.find_by(id: params[:id])
if #activity.update_attributes(activity_params)
flash[:sucess] = "Class updated!"
redirect_to edit_activity_path(#activity)
else
render 'edit'
end
end
...
private
def activity_params
params.require(:activity).permit(:name, :active, #active is set to default: true
:timeslots_attributes => [:id,
:time_start,
:time_end,
:day,
:active])
end
end
But whenever I try to create a new activity I get the error message "Timeslots activities must exist".
I feel it's trying to assign the activity_id for timeslot before the activity is created, but I'm not sure. I've tried many things (some of which I've included in my example in comment form) but am unable to work out why I'm getting this error.
Update: Add error log
Started POST "/activities" for 127.0.0.1 at 2016-09-25 18:04:51 +0700
Processing by ActivitiesController#create as HTML
Parameters: {"utf8"=>"✓", "authenticity_token"=>"xp+dBcWC4cjI6FLpIqhU0RzM4ldZ4JpkFLSyAXcmifL73QqWz6R65EHm/Tj7QxlXnWiBA0axjVXvMZHQ+XKA9A==", "activity"=>{"name"=>"Newest class", "timeslots_attributes"=>{"0"=>{"time_start(1i)"=>"2016", "time_start(2i)"=>"9", "time_start(3i)"=>"25", "time_start(4i)"=>"12", "time_start(5i)"=>"00", "time_end(1i)"=>"2016", "time_end(2i)"=>"9", "time_end(3i)"=>"25", "time_end(4i)"=>"13", "time_end(5i)"=>"30", "day"=>"4"}}}, "commit"=>"Create class"}
Club Load (0.2ms) SELECT "clubs".* FROM "clubs" WHERE "clubs"."id" = ? LIMIT ? [["id", 2], ["LIMIT", 1]]
(0.1ms) begin transaction
(0.1ms) rollback transaction
Rendering activities/new.html.erb within layouts/application
Rendered shared/_error_messages.html.erb (1.5ms)
Rendered activities/new.html.erb within layouts/application (16.9ms)
Rendered layouts/_rails_default.html.erb (58.5ms)
Rendered layouts/_shim.html.erb (0.5ms)
Rendered layouts/_header.html.erb (1.7ms)
Rendered layouts/_footer.html.erb (0.8ms)
Completed 200 OK in 111ms (Views: 93.9ms | ActiveRecord: 0.3ms)

Answer 1

You're getting this error because Rails 5 makes belongs_to attribute required by default. Also, the saving mechanism kind of follows:
validate your parent model
validate your child model # validation fails here because parent doesn't have an id yet, because it hasn't been saved
save parent model
save child model
A way to resolve this is to:
class Activity < AR
has_many :timeslots, dependent: :destroy, inverse_of: :activity
accepts_nested_attributes_for :timeslots
end
class Timeslot < AR
belongs_to :activity, inverse_of: :timeslot
end
You could learn more of this here and here