I have tried all of the solutions to similar problems and haven't gotten this one figured out.
I have a has_many :through relationship between 'Clinician', and 'Patient' with a joined model 'CareGroupAssignment'.  I would like to have a patient be able to have multiple clinicians associated with it and clinicians will have multiple patients.
When I submit my form this is logged:
Started POST "/patients" for 127.0.0.1 at 2015-09-02 14:56:38 -0700
Processing by PatientsController#create as HTML
  Parameters: {"utf8"=>"✓", "authenticity_token"=>"XXX=", "patient"=>{"user_attributes"=>{"email"=>"allencentre", "password"=>"[FILTERED]"}, "first_name"=>"Allen", "last_name"=>"Centre", "birth_date(1i)"=>"2002", "birth_date(2i)"=>"9", "birth_date(3i)"=>"2", "clinician_ids"=>["85", "87"], "patient_deceased"=>"0", "patient_archived"=>"0"}, "button"=>""}
So "clinician_ids"=>["85", "87"] is being passed from the form.
There is an INSERT INTO "users" ("email", .... and an INSERT INTO "patients" ("bir.... but the data for care_group_assignments doesn't have one.
There is [36mCareGroupAssignment Load (0.4ms)[0m  [1mSELECT "care_group_assignments".* FROM "care_group_assignments"  WHERE "care_group_assignments"."patient_id" = $1[0m  [["patient_id", 199]]
clinician.rb (simplified)
class Clinician < ActiveRecord::Base
    belongs_to :care_group
    has_many :patients ,:through=> :care_group_assignments
    has_many :care_group_assignments, :dependent => :destroy
    belongs_to :user
    accepts_nested_attributes_for :user,  :allow_destroy => true
end
patient.rb
class Patient < ActiveRecord::Base
    belongs_to :care_group
    has_many :clinicians ,:through=> :care_group_assignments
    has_many :care_group_assignments
    belongs_to :user
    accepts_nested_attributes_for :user,  :allow_destroy => true
end
care_group_assignments.rb
class CareGroupAssignment < ActiveRecord::Base
    belongs_to :clinician
    belongs_to :patient
end
This is following the example from Railscasts PRO #17- HABTM Checkboxes to at least start getting the data collected and to have the models set up correctly. Below is the form with the checkboxes for each clinician as described in the RailsCast, checkboxes show up and the data is sent but not stored (can't figure out why).
patient new.html.erb form
<%= form_for @patient do |form| %>
  <%= form.fields_for :user do |builder| %>
   <div class="form-group">
      <%= builder.label "Email or Username" %>
      <%= builder.text_field :email, class: "form-control" %>
    </div>
    <div class="form-group">
      <%= builder.label :password %>
      <%= builder.password_field :password, class: "form-control" %>
    </div>
  <% end %>
  <div class="form-group">
    <%= form.label :first_name %>
    <%= form.text_field :first_name, class: "form-control", placeholder: "First name" %>
  </div>
  <div class="form-group">
    <%= form.label :last_name %>
    <%= form.text_field :last_name, class: "form-control", placeholder: "Last name" %>
  </div>
  <div class="form-group">
    <% Clinician.where(care_group_id: @care_group.id).each do |clinician| %>
      <%= check_box_tag "patient[clinician_ids][]", clinician.id, @patient.clinician_ids.include?(clinician.id), id: dom_id(clinician) %>
      <%= label_tag dom_id(clinician), clinician.full_name %><br>
    <% end %>
  </div>
  <%= form.button 'Create Patient', class: "btn btn-u btn-success" %>
<% end %>
This method has so far not been able to save the clinician to patient association.
patient_controller.rb
def new
  @patient = Patient.new
  @user = User.new
  @patient.build_user
  @care_group = current_clinician.care_group
end
def create
  @patient = Patient.create(patient_params)
  @patient.care_group = current_clinician.care_group
  if @patient.save
    redirect_to patient_path(@patient), notice: "New patient created!"
  else
    render "new"
  end
end
def show
 @patient = Patient.find_by(id: params["id"])
end
private
 def patient_params
  params.require(:patient).permit({:clinician_ids => [:id]},:first_name,:last_name,:user_id,:birth_date, user_attributes: [ :email, :password, :patient_id, :clinician_id ])
 end
I plan to display the clinicians associated with a patient on the patient show page:
patient show.html.erb
<strong>Shared with:</strong>
 <% @patient.clinicians.each do |clinician| %>
  <%= clinician.full_name %><
 <% end %>
This works if I seed the database but since the data doesn't seem to be stored, nothing is showing up.
Rails 4.1.8, ruby 2.2.1p85, PostgreSQL
Thanks