controller

  # GET /allergies/new
  # GET /allergies/new.json
  def new
    @patient = Patient.find(params[:patient_id])
    @allergy = @patient.allergies.new


    respond_to do |format|
      format.html # new.html.erb
      format.json { render json: @allergy }
    end
  end

  # GET /allergies/1/edit
  def edit
    @allergy = Allergy.find(params[:id])
  end

  # POST /allergies
  # POST /allergies.json
  def create
    #@allergy = Allergy.new(params[:allergy])
    @allergy = Patient.find(params[:patient_id]).allergies.build(params[:allergy])

    respond_to do |format|
      if @allergy.save
        format.html { redirect_to patient_url(params[:patient_id]), notice: 'Allergy was successfully created.' }
        #format.json { render json: @allergy, status: :created, location: @allergy }
      else
        format.html { render action: "new" }
        format.json { render json: @allergy.errors, status: :unprocessable_entity }
      end
    end
  end

error

AllergiesControllerTest:
     FAIL should create allergy (0.40s) 
          "Allergy.count" didn't change by 1.
<3> expected but was
<2>.

test

  test "should create allergy" do
    assert_difference('Allergy.count') do
      post :create, :post => @allergy, :patient_id => @allergy.patient_id
    end
    assert_redirected_to patients_path
  end