個人開発のサービスを売買できる「個人開発のフリマ」を作りました

class User < ApplicationRecord
  has_many :businesses, dependent: :destroy

  has_many :buy_negotiations,  class_name: "Negotiation",
                                  foreign_key: "buyer_id",
                                  dependent: :destroy
  has_many :sell_negotiations, class_name: "Negotiation",
                                  foreign_key: "seller_id",
                                  dependent:   :destroy
  has_many :buy_content, through: :buy_negotiations,  source: :buyer
  has_many :sell_content, through: :sell_negotiations, source: :seller
  has_many :messages
end

class Business < ApplicationRecord
  belongs_to :user
  has_many :negotiations
end

class Negotiation < ApplicationRecord
  before_create :set_slug

  belongs_to :business
  has_many :messages

  belongs_to :buyer, class_name: "User"
  belongs_to :seller, class_name: "User"

  validates :buyer_id, presence: true
  validates :seller_id, presence: true

  def to_param
    slug
  end

  private

  def set_slug
    loop do
      self.slug = SecureRandom.uuid
      break unless Negotiation.where(slug: slug).exists?
    end
  end
end

class Message < ApplicationRecord
  belongs_to :user
  belongs_to :negotiation
end

<%= form_with model: @negotiation, url: negotiations_path, local: true do |f| %>
  <%= f.hidden_field :business_id, value: @business.id %>
  <%= f.hidden_field :seller_id, value: @business.user.id %>
  <%= f.hidden_field :buyer_id, value: current_user.id %>
  <%= f.submit "売却の交渉を始める", class: "btn btnPrimary btnStyle" %>
<% end %>

  def show
・・・
    @negotiation = @business.negotiations.build
  end

  def create
    @negotiation = Negotiation.create(create_params)
    if @negotiation.save
      NegotiationMailer.send_notification_to_seller(@negotiation).deliver
      flash[:success] = "交渉が始まりました！"
      redirect_to @negotiation
    else
      flash[:alert] = "交渉の開始に失敗しました。"
      redirect_to root_path
    end
  end

・・・
    <div class="chatBox">
      <%= render partial: 'messages/form', locals: { message: @message, negotiation: @negotiation, negotiation_id: @negotiation.id } %>
    </div>
    <div id="comments_area">
       <%= render partial: 'messages/index', locals: { messages: @messages } %>
    </div>
・・・

<%= form_with(model: [negotiation, message] ) do |form| %>
  <div class="chatInput">
    <%= form.text_area :content, rows: "3" %>
  </div>
  <div class="chatHead">
    <%= form.submit "送信する" %>
  </div>
<% end %>

<ul>
  <% messages.each do |message| %>
    <% unless message.id.nil? %>
      <li>
        <div class="msg">
          <div class="flexContainer">
            <div class="icon">
              <%= image_tag message.user.image.to_s %>
            </div>
            <div class="text">
              <div class="name">
                <span><%= message.user.username %></span>
              </div>
              <div class="content">
                <p><%= message.content %></p>
              </div>
            </div>
          </div>
        </div>
      </li>
    <% end %>
  <% end %>
</ul>

$("#comments_area").html("<%= j(render 'index', { messages: @message.negotiation.messages }) %>")
$("textarea").val('')

  def show
    @negotiation = Negotiation.find_by_slug(params[:slug])
    @message = Message.new
    @messages = @negotiation.messages
  end

  def create
    @message = @negotiation.messages.build(create_params)
    @message.user_id = current_user.id
    if @message.save
      MessageMailer.send_notification_to_receiver(@message).deliver
      render :index
    end
  end