How to create triple nested objects in Rails with multiple-directional associations - ruby-on-rails

Question

An article called Triple nested Forms in Rails presents a good description of creating a form for saving three nested objects. The example given is of creating a Show that has_many Seasons, and each Season has_many Episodes. Also, Episode --> belongs_to --> Season --> belongs_to --> Show. 
Shows are created like this:
def new
#show = Show.new
#show.seasons.build.episodes.build
end
The form looks like this:
<%= form.fields_for :seasons do |s| %>
    <%= s.label :number %>
    <%= s.number_field :number %>    <%= s.fields_for :episodes do |e| %>
      <%= e.label :title %>
      <%= e.text_field :title %>
    <% end %> 
 <% end %>
<% end %>
This seems straightforward because all the associations run in one direction. I'm trying to do something that's similar, but more complicated. I have a Parent model where each Parent has multiple Children and each Child is enrolled in a School. After specifying that Children is the plural of Child, the association would have to be like this:
Parent has_many Children, accepts_nested_attributes_for :children
Child belongs_to Parent, belongs_to School, accepts_nested_attributes_for :school
School has_many Children, accepts_nested_attributes_for :children
Graphically, it would look like this:
Parent <-- belongs_to <-- Child --> belongs_to --> School
Each Parent is also associated with a User, like this:
User has_many :parents
The data on Parents, Children, and Schools is entered in the following form (generated using the Simple Form gem), where the schools are entered as a dropdown selector populated from the schools table:
#schools = School.all
<%= simple_form_for (#parent) do |f| %>
     <%= f.input :name, label: 'name' %>
     <%= f.simple_fields_for :children, #children do |child_form| %>
     <%= child_form.input :name, label: "Child Name" %>
         <%= child_form.simple_fields_for :school, #school do |school %>
     <%= school.collection_select :id, #schools, :id, :name, {}, {} %>
        <% end %>
     <% end %>
<% end %>
I set up the new controller method to create a Parent having three Children enrolled in an existing School. Then I tried to associate the Children with a School that already exists in the schools table with id = 1.
def new
#parent = Parent.new
# creating 3 children
#children = Array.new(3) {#parent.children.build}
#school = School.find(1)
#school.children.build
end
This throws an error 
Couldn't find School with ID=1 for Child with ID=
The error is located in the first line of the create method, which looks like this:
def create
#parent = Parent.new(parent_params.merge(:user => current_user))
if #parent.save
redirect_to root_path
else
render :new, :status => :unprocessable_entity
end
end
def parent_params
params.require(:parent).permit(:name, :child_attributes => [:id, :name, age, :school_attributes => [:id, :name]])
end
Since the error text states "Child with ID= ", the error must be thrown before ids for new Children are assigned. Why can a School with ID=1 not be found when it exists in the schools table? Or, does this mean that a School record has not been properly associated with an instance of Child before an attempt is made to save that instance? If so, how can I fix the association?

Answer 1

One of the most common missconceptions/misstakes with nested attributes is to think that you need it to do simple association assignment. You don't. You just need to pass an id to the assocation_name_id= setter.
If fact using nested attributes won't even do what you want at all. It won't create an assocation from an existing record when you do child.school_attributes = [{ id: 1 }] rather it will attempt to create a new record or update an existing school record.
You would only need to accept nested attributes for school if the user is creating a school at the same time. And in that case its probally a better idea to use Ajax rather than stuffing everything into one mega action.
<%= simple_form_for (#parent) do |f| %>
<%= f.input :name, label: 'name' %>
<%= f.simple_fields_for :children, #children do |child_form| %>
<%= child_form.input :name, label: "Child Name" %>
<%= child_form.associaton :school,
collection: #schools, label_method: :name %>
<% end %>
<% end %>
def parent_params
params.require(:parent).permit( :name,
child_attributes: [:id, :name, :age, :school_id]]
)
end