ruby初心者です。
fields_forの親子関係が逆の時はどうすればいいですか。
coordinationの外部キーにouterがあります。
coordinationは
belongs_to :outer, optional: true
outerは
has_many :coordinations
accepts_nested_attributes_for :coordinations
となっていますがcoordinationに保存しますが、親子関係はそれの逆となっております。

解決したいこと
coordination.controllerの@coordination.outer.buildの部分が
undefined method `build' for nil:NilClass を起こします。 .newにしても同様にエラーが起きます。

.buildを外したら、エラー無く操作できますが外部キーにcheck_ boxで選択したouterの画像を保存できません。

おそらく正則化などが問題としてあるのかなと考えていますが、どなたかご教授お願いします。

coordination.controller

class CoordinationsController < ApplicationController
  before_action :coordination_params ,only:[:create]
  def index
    @coordination = Coordination.new
  end

  def new
    @coordinations = Coordination.all
    @outers = Outer.all
    @inners = Inner.all
    @bottoms = Bottom.all
    @shoes = Shoe.all
    @hats = Hat.all
    @accessories = Accessory.all
    @coordination = Coordination.new
    @coordination.outer.build
    @outer = Outer.new
    @inner = Inner.new
    @bottom = Bottom.new
    @shoe = Shoe.new
    @hat = Hat.new
    @accessory = Accessory.new
  end

  def create
    @coordination = Coordination.create!(coordination_params)
  end
  def show
    @coordinaitions = Coordination.all
  end

  def edit
    @coordinations = Coordination.find(params[id])
    @outers = Outer.find(params[id])
    @inners = Inner.find(params[id])
    @bottoms = Bottom.find(params[id])
    @shoes = Shoe.find(params[id])
    @hats = Hat.find(params[id])
    @accessories = Accessory.find(params[id])
  end

  def update

    @coordinations = Coordination.find(params[id])
    @inners = Inner.find(params[id])
    @bottoms = Bottom.find(params[id])
    @shoes = Shoe.find(params[id])
    @hats = Hat.find(params[id])
    @accessories = Accessory.find(params[id])
  end

  private
  def coordination_params
    params.require(:coordination).permit(:season, :coordination, :inner_id, :bottom_id, :shoes_id, :hat_id, :accessory_id, outer_attributes: [:outer]).merge(user_id: current_user.id)
  end
end

outer.controller

class OutersController < ApplicationController

  def index
    @outers = Outer.all
  end
  def new 
    @outer = Outer.new
  end

  def create
    Outer.create(outer_params)
  end

  def edit
    @outers = Outer.find(params[id])
  end
  def update

    @outers = Outer.find(params[id])
  end
  def destroy
    outer = Outer.find(params[:id])
    outer.destroy
  end

  private
  def outer_params
    params.require(:outer).permit(:outer, coordination_attributes: [:outer_id])
  end
end

coordination.model

class Coordination < ApplicationRecord
  belongs_to :outer, optional: true
  belongs_to :inner, optional: true
  belongs_to :bottom, optional: true
  belongs_to :shoe, optional: true
  belongs_to :hat, optional: true
  belongs_to :accessory, optional: true
  belongs_to :user, optional: true
  has_many :accessory_coordinations
  mount_uploader :outer_id, ImageUploader
end

outer.model

class Outer < ApplicationRecord
  belongs_to :user, optional: true
  has_many :coordinations
  mount_uploader :outer, ImageUploader
  accepts_nested_attributes_for :coordinations
end

coordination/new.haml

.lists
  .cloth
    服一覧
    = form_for @outer do |f|
      = f.label :"アウター", class: "outer-btn"
      = f.file_field :outer, class: "file-btn"
      = f.submit "追加", class: "add-btn"
      %br

      .bbbbb
        V
        - @outers.each do |abc|
          = f.fields_for :coordination do |q|
            = image_tag (abc.outer), class: 'apapap'
            -# = q.label :outer, class: "apapap"
            = q.check_box :outer_id, class: "apapap"
            = q.submit "送信", class: "apapap"

coordination.migrate

class CreateCoordinations < ActiveRecord::Migration[5.0]
  def change
    create_table :coordinations do |t|
      t.string :season
      t.string :coordination
      t.timestamps
    end
  end
end

outer.migrate

class CreateOuters < ActiveRecord::Migration[5.0]
  def change
    create_table :outers do |t|
      t.string :outer
      t.string :image
      t.timestamps
    end
  end
end

coordination.addreference.migrate

class CreateChageCoordinations < ActiveRecord::Migration[5.0]
  def change
    create_table :chage_coordinations do |t|
      add_reference :coordinations, :outer, foreign_key: true
      add_reference :coordinations, :inner, foreign_key: true
      add_reference :coordinations, :bottom, foreign_key: true
      add_reference :coordinations, :shoes, foreign_key: true
      add_reference :coordinations, :hat, foreign_key: true
      add_reference :coordinations, :accessory, foreign_key: true
      t.timestamps
      t.timestamps
    end
  end
end