fields_for not saving nested parameters in rails 5.0.1 - nested

Question

I'm fairly new to rails, though I've made some basic apps and read several tutorials, this is the first time I've delved into nested attributes inside forms and the use of fields_for.
I've searched trough many similar questions and have read the documentation on fields_for, however my form doesn't seem to save whenever I use fields_for inside my form. I've tested without the fields_for and saves as it should (without saving to the nested DB, obviously).
What I'm trying to do is a simple Event registration, which in turn has some attributes and a date in a separate model.
I've tried with and without validations, many different things inside the strong parameters
Here is my Event models:
class Event < ApplicationRecord
has_many :eventdates, dependent: :destroy
accepts_nested_attributes_for :eventdates
validates :name, :description, :city, :street, :postal_code, presence: true
end
The Eventdate model:
class Eventdate < ApplicationRecord
belongs_to :event
validates :date, :start_hour, :finish_hour, :event_id, presence: true
end
The Event controller:
class EventsController < ApplicationController
def new
#event = Event.new
#event.eventdates.build
end
def create
#event = Event.new(event_params)
if #event.save
redirect_to root_url
else
render "static_pages/home"
end
end
private
def event_params
params.require(:event).permit(:name, :description, :street, :city, :postal_code, :address_number, :additional_info,
eventdates_attributes: [:id, :date, :start_hour, :finish_hour, :event_id])
end
And the form:
<%= form_for(#event, url: events_path) do |f| %>
<p>
<%= f.label :nombre %>
<%= f.text_field :name %>
</p>
<div>
<%= f.fields_for :eventdates do |eventdate_fields| %>
<%= eventdate_fields.label :fecha %>
<%= eventdate_fields.date_field :date %>
<%= eventdate_fields.label :hora_de_inicio %>
<%= eventdate_fields.time_field :start_hour %>
<%= eventdate_fields.label :hora_de_termino %>
<%= eventdate_fields.time_field :finish_hour %>
<% end %>
</div>
<p>
<%= f.label :infomación_adicional %>
<%= f.text_area :additional_info %>
</p>
<p>
<%= f.submit "Crear evento" %>
</p>
<% end %>
I'm pretty sure this is a very simple form, but somehow refuses to save at all into the database.

Answer 1

In eventdate.rb model, append optional: true to this line: belongs_to :event
class Eventdate < ApplicationRecord
belongs_to :event, optional: true
validates :date, :start_hour, :finish_hour, :event_id, presence: true
end
Because Rails will create eventdates first, then create event and links to those evendates (by updating all event_id in these evendates). But when creating eventdates, column event_id is nil and eventdates cannot be saved.