Rails 5: Is Nested forms set up correct - ruby

Question

If I want a Nested Form for volunteer_shift
inside my assignment/_form
How should my model associations be set up?
My current associations look like this..but I am not sure they are correct if the volunteer_shift partial sits inside the assignment form....
When I try to open the "add new record" form...my assignment#new action errors out...
error message
Completed 500 Internal Server Error in 30ms (ActiveRecord: 2.7ms)
NoMethodError - undefined method `build' for nil:NilClass:
app/controllers/assignments_controller.rb:21:in `new'
Here is my new action...
# GET /assignments/new
def new
#assignment = Assignment.new
# binding.pry
#assignment.volunteer_event.build <----Line 21
#my_url = {:action => "create", :id => params[:id]}
end
Here are my current models associations...
class Assignment < ApplicationRecord
attr_accessor :volunteer_event
belongs_to :volunteer_shift
has_one :volunteer_task_type, :through => :volunteer_shift, :source => :volunteer_task_type
belongs_to :contact ,optional: true
validates_presence_of :volunteer_shift #belongs_to takes care of this now
validates_associated :volunteer_shift
accepts_nested_attributes_for :volunteer_shift, allow_destroy: true
...
class VolunteerShift < ApplicationRecord
has_many :assignments
belongs_to :volunteer_event
...
class VolunteerEvent < ApplicationRecord
belongs_to :volunteer_default_event
has_many :volunteer_shifts, :dependent => :destroy
has_many :resources_volunteer_events, :dependent => :destroy
validates_associated :volunteer_shifts
And inside my assignment/form
<%= form_for #assignment, :url => #my_url, remote: true do |f| %>
...
<!--VOLUNTEER SHIFT-->
<!--TODO: make this a partial under field_for-->
<%= f.fields_for :volunteer_shift do |builder| %>
<%= render 'volunteer_shift_fields', vs: builder %>
<% end %>
...
<% end %>
and if you want to see the volunteer_shift_fields partial
<div class="name large flex-row">
<%= vs.label :volunteer_shift %>
</div>
<div id="volunteer_shift" class="d-flex flex-row">
<div class="col-sm-12 p-2">
<div id="volunteer_shift" class="text-right">
<!-- old if: if class is assignment show volunteer shift else show default shift -->
<!-- we need default shift here...NO assignment is attached-->
<div class="field">
<%= vs.label :volunteer_task_type_id %>
<%= select_tag 'volunteer_task_type_id', options_from_collection_for_select([VolunteerTaskType.new(:description => ""), VolunteerTaskType.instantiables.effective_on(Date.today)].flatten, "id", "description") %>
</div>
<div class="field">
<%= vs.label :roster_id %>
<%= select_tag 'roster_id', options_from_collection_for_select([Roster.new(:name => ""), Roster.all].flatten, "id", "name") %>
</div>
<div class="field">
<%= vs.label :program_id %>
<%= select_tag 'program_id', options_from_collection_for_select([Program.new(:name => ""), Program.where(:volunteer => true)].flatten, "id", "name")%>
</div>
<div class="field">
<%= vs.label :set_description %>
<%= vs.text_field(:set_description, nil) %>
</div>
<div class="field">
<%= vs.label :set_date, "Date" %> <
<%= vs.text_field(:set_date, nil) %>
</div>
</div>
</div>
</div>
Are my model associations correct?
What is my "assignment#new" action missing?

Answer 1

Remove attr_accessor :volunteer_event, you need to set up the model association to volunteer_event through volunteer_shift
so
class Assignment < ApplicationRecord
attr_accessor :volunteer_event
needs to become
class Assignment < ApplicationRecord
has_one :volunteer_event, through: :volunteer_shift
Looking at your volunteer_shift associations I'm pretty sure that you only want to ever have a single volunteer_event record for a single assignment? if that's not what you want then you need to look at your associations again
Or...
You could change the attr_accessor to attr_reader and assign the relevant volunteer_event record in an initializer but I would not recommend that approach for this case