Undefined method 'constant' error

  class PostsController < ApplicationController
   before_action :set_post, only: [:show, :edit, :update, :destroy]
   before_action :correct_user, only: [:edit, :update, :destroy]
   before_action :authenticate_user!, except: [:index, :show]

  def index
    @posts = Post.all.order("created_at DESC").paginate(:page =>     params[:page], :per_page => 25)
   end

  def show
  end

  def new
    @post = current_user.posts.build
  end

  def edit
  end

  def create
    @post = current_user.posts.build(post_params)

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

  def update
    respond_to do |format|
      if @post.update(post_params)
        format.html { redirect_to @post, notice: 'Post was successfully updated.' }
        format.json { head :no_content }
      else
        format.html { render action: 'edit' }
        format.json { render json: @post.errors, status: :unprocessable_entity }
      end
    end
  end

  def destroy
    @post.destroy
    respond_to do |format|
      format.html { redirect_to posts_url }
      format.json { head :no_content }
    end
  end

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

    def correct_user
      @post = current_user.posts.find_by(id: params[:id])
      redirect_to posts_path, notice: "Not authorized to edit this post" if @post.nil?
    end

    # Never trust parameters from the scary internet, only allow the white list through.
    def post_params
      params.require(:post).permit(:description, :image)
    end
end

<div class="page-header">
  <center><strong><h1> Explore Page </h1></strong></center>
</div>

  <% if user_signed_in? %>
    <center><%= link_to 'New Post', new_post_path, class: "btn btn-danger btn-lg active" %></center>
  <% end %>  

  <div id="posts" class="transitions-enabled">
      <% @posts.each do |post| %>

        <div class="box panel panel-default">
       <%= link_to image_tag(post.image.url(:medium)), post %>
      <div class="panel-body">
      <%= post.description %><br/>
      <strong><%= post.user.name if post.user %></strong>

      <% if post.user == current_user %>
        <div class="actions">
        <%= link_to edit_post_path(post) do %>
        <span class="glyphicon glyphicon-edit"></span>
            Edit
          <% end %>
        <%= link_to post, method: :delete, data: { confirm: 'Are you sure?' } do %>
        <span class="glyphicon glyphicon-trash"></span>
            Delete
          <% end %>
        </div>
       <% end %>
      </div>
      </div> 
   <% end %>
  </div>

 <div class="center">
   <%= will_paginate @posts, renderer: BootstrapPagination::Rails %>
 </div>

 class Post < ActiveRecord::Base

belongs_to :user

has_attached_file :image, :styles => { :medium => "300x300>", :thumb => "100x100>" }

validates_attachment_content_type :image, :content_type => ["image/jpg", "image/jpeg", "image/png"]

validates :description, presence: true
validates :image, presence: true

# Returns microposts from the users being followed by the given user.
  def self.from_users_followed_by(user)
    followed_user_ids = user.followed_user_ids
    where("user_id IN (:followed_user_ids) OR user_id = :user_id",
      followed_user_ids: followed_user_ids, user_id: user)
    end
  end