Rails nested fields_for duplicate records in DB

1. Question is how to refactor _form.html.erb to properly save all data to the db without duplicating entrie.
2. The problem is when trying to create new row in a valuation line rails save in one row valuation_id, part_id and another row valuation_id and pruchase_price. Kind of duplicating rows whereas it should merge all this data in one row.

The situation is I have a valuation model which has many valuation_lines. The goal of this program is to prepare a valuation (offer) with multiple parts (items on it). I am using a join table valuation_lines which stores every line of differrent valuations. (To keep things simple at this moment i have a form with only one valuation line).

ps. I went through all railscast on nested models... :(

My valuation form looks like:

   <%= form_for(@valuation) do |f| %>
  <% if @valuation.errors.any? %>
    <div id="error_explanation">
      <h2><%= pluralize(@valuation.errors.count, "error") %> prohibited this valuation from being saved:</h2>

      <ul>
      <% @valuation.errors.full_messages.each do |msg| %>
        <li><%= msg %></li>
      <% end %>

      </ul>
    </div>
  <% end %>

  <div class="field">
    <%= f.label :description %><br>
    <%= f.text_area :description %>
  </div>
<div class="field">

</div>
<div class="field">

<%= f.fields_for :valuation_lines do |builder| %>

<%= f.collection_select(:part_ids, @parts, :id, :name) %><br/>
      <%= builder.label :pruchase_price, "Price:" %> <br/>
      <%= builder.text_field :pruchase_price %>
<% end %>

</div>    

<div class="field">
<br>  
  <div class="actions">
    <%= f.submit %>
  </div>
<% end %>

valuations_controller.rb

class ValuationsController < ApplicationController
  before_action :set_valuation, only: [:show, :edit, :update, :destroy]

  # GET /valuations
  # GET /valuations.json
  def index
    @valuations = Valuation.all
  end

  # GET /valuations/1
  # GET /valuations/1.json
  def show
  end

  # GET /valuations/new
  def new
    @valuation = Valuation.new
    @parts = Part.all
    @valuation.valuation_lines.build
  end

  # GET /valuations/1/edit
  def edit
  end

  # POST /valuations
  # POST /valuations.json
  def create
    @valuation = Valuation.new(valuation_params)

    respond_to do |format|
      if @valuation.save
        format.html { redirect_to @valuation, notice: 'Valuation was successfully created.' }
        format.json { render action: 'show', status: :created, location: @valuation }
      else
        format.html { render action: 'new' }
        format.json { render json: @valuation.errors, status: :unprocessable_entity }
      end
    end
  end

  # PATCH/PUT /valuations/1
  # PATCH/PUT /valuations/1.json
  def update
    respond_to do |format|
      if @valuation.update(valuation_params)
        format.html { redirect_to @valuation, notice: 'Valuation was successfully updated.' }
        format.json { head :no_content }
      else
        format.html { render action: 'edit' }
        format.json { render json: @valuation.errors, status: :unprocessable_entity }
      end
    end
  end

  # DELETE /valuations/1
  # DELETE /valuations/1.json
  def destroy
    @valuation.destroy
    respond_to do |format|
      format.html { redirect_to valuations_url }
      format.json { head :no_content }
    end
  end

  private
    # Use callbacks to share common setup or constraints between actions.
    def set_valuation
      @valuation = Valuation.find(params[:id])
    end

    # Never trust parameters from the scary internet, only allow the white list through.
    def valuation_params
       params.require(:valuation).permit(:description, :part_id, :valuation_id, :pruchase_price, :valuation_line, :quantity, :part_ids, parts_attributes: [:id, :code, :name], valuation_lines_attributes: [:id, :valuation_id, :part_id, :pruchase_price, :quantity])

  end
end

Valuation Model

 class Valuation < ActiveRecord::Base
        has_many :valuation_lines
        has_many :parts, :through => :valuation_lines
        accepts_nested_attributes_for :parts
        accepts_nested_attributes_for :valuation_lines

    end

Valuation_line model

class ValuationLine < ActiveRecord::Base
    belongs_to :part
    belongs_to :valuation
end

Part model

class Part < ActiveRecord::Base
    has_many :valuation_lines
    has_many :valuations, :through => :valuation_lines
end

schema_db:

  create_table "parts", force: true do |t|
    t.string   "code"
    t.string   "name"
    t.datetime "created_at"
    t.datetime "updated_at"
  end

  create_table "valuation_lines", force: true do |t|
    t.integer "valuation_id"
    t.integer "part_id"
    t.integer "pruchase_price"
    t.integer "quantity"
  end

  create_table "valuations", force: true do |t|
    t.text     "description"
    t.datetime "created_at"
    t.datetime "updated_at"
  end

end

development log after adding new valuation returns:

Started POST "/valuations" for 127.0.0.1 at 2014-08-20 22:08:46 +0200
Processing by ValuationsController#create as HTML
  Parameters: {"utf8"=>"✓", 
"authenticity_token"=>"6ADrGOl39bdD3FExQj0l405SXKgJ2zwCqVi6p+JQFw4=", "valuation"=>{"description"=>"33", "part_ids"=>"1"}, "valuation_lines"=>{"pruchase_price"=>"888"}, "commit"=>"Create Valuation"}
  [1m[36mPart Load (38.0ms)[0m  [1mSELECT "parts".* FROM "parts" WHERE "parts"."id" = ? LIMIT 1[0m  [["id", 1]]
  [1m[35m (0.0ms)[0m  begin transaction
  [1m[36mSQL (29.0ms)[0m  [1mINSERT INTO "valuations" ("created_at", "description", "updated_at") VALUES (?, ?, ?)[0m  [["created_at", Wed, 20 Aug 2014 20:08:46 UTC +00:00], ["description", "33"], ["updated_at", Wed, 20 Aug 2014 20:08:46 UTC +00:00]]
  [1m[35mSQL (0.0ms)[0m  INSERT INTO "valuation_lines" ("part_id", "valuation_id") VALUES (?, ?)  [["part_id", 1], ["valuation_id", 59]]
  [1m[36m (76.0ms)[0m  [1mcommit transaction[0m
Redirected to http://localhost:3000/valuations/59
Completed 302 Found in 262ms (ActiveRecord: 143.0ms)