初めての質問です。
Ruby on railsで勉強をさせて頂いているのですが
rails sを行いページを見ようとすると下記のようなエラがー出てしまいます。

解決法をお教え頂ければ幸いです。

  • ruby v2.2.2
  • rails v4.2.5.1

エラー名

NoMethodError in ProjectController#show 
undefined method `users' for #<Project:0x007f9ef37c73a8>

app/controllers/project.controller.rb

class ProjectController < ApplicationController

  before_action :authenticate_user!, only: [:list]

  def index
    @projects = Project.all
  end

  def show
    @project = Project.find(params[:id])
    @tasks = @project.tasks.order(:tag)

    @joined = false

    if !current_user.nil? && !current_user.projects.nil?
      @joined = current_user.projects.include?(@project)
    end

    @users = @project.users.order('created_at desc').first(10)
  end

  def list    
    if !current_user.nil?
      @projects = current_user.projects
    end
  end
end

app/admin/project.rb

ActiveAdmin.register Project do
  permit_params :name, :content, :price, :image

  show do |t|
    attributes_table do
      row :name
      row :content
      row :price
      row :image do
        project.image? ? image_tag(project.image.url, height: '100') : content_tag(:span, "No photo yet")
      end
    end
  end

  form :html => { :enctype => "multipart/form-data" } do |f|
    f.inputs do
      f.input :name
      f.input :content
      f.input :price  
      f.input :image, hint: f.project.image? ? image_tag(project.image.url, height: '100') : content_tag(:span, "Upload JPG/PNG/GIF image")
    end
    f.actions
  end
end

app/model/project.rb

    class Project < ActiveRecord::Base
  extend FriendlyId
  friendly_id :name, use: [:slugged, :finders]

  has_many :tasks

  validates :name, presence: true, length: { maximum: 50 }
  validates :content, presence: true, length: { maximum: 500 }
  validates :price, presence: true, numericality: { only_integer: true }

  has_attached_file :image, :styles => { :medium => "680x300>", :thumb => "170x75>" }
  validates_attachment_content_type :image, :content_type => /\Aimage\/.*\Z/

  def shortname
    name.length > 25? name[0..25] + "..." : name
  end
end

app/project/views/project/show.html.rb