Add multiple images to a model using Paperclip and active_admin

admin-projects.rb

ActiveAdmin.register Project do
  controller do
    def permitted_params
      params.permit project: [:title, :summary, :description, :thumbnail, :date, :url, :client_list, :technology_list, :type_list, :images_attributes => [:picture, :id, :_destroy]]
      # to add unlimited :image, you need to permit :images_attributes => [:picture, :id, :_destroy]
      #  - picture for the image
      #  - id to avoid duplicates
      #  - _destroy if you want to allow image delete
    end
  end

  form do |f|
    f.inputs "Project Details" do
      f.input :title
      f.has_many :images do |p|
        p.inputs do
          p.input :_destroy, :as => :boolean, :label => "Destroy?" unless p.object.new_record?
          # add a _destroy field if this is an existing record (don't forget to permit this field)
          p.input :picture, :as => :file, :hint => p.object.new_record? ? "" : f.template.image_tag(p.object.picture.url(:thumb))
          # add a new picture and display the image if the picture already exists
        end
      end
    end
    f.actions
  end

  show do |ad|
      attributes_table do
        row :title
        # Loop on every existing images and display them in a list
        row :images do
          ul do
            ad.images.each do |img|
              li do
                image_tag(img.picture.url(:thumb))
              end
            end
           end
        end
      end
    end
end

migrate-create_image.rb

class CreateImages < ActiveRecord::Migration
  def change
    create_table :images do |t|
      t.belongs_to  :project
      t.attachment  :picture
      t.timestamps
    end
  end
end

model-image.rb

class Image < ActiveRecord::Base
  belongs_to :project

  has_attached_file :picture, :styles => { :large => "960x640>", :medium => "300x300>", :thumb => "150x150>" }, :default_url => "/images/:style/missing.png"
end

models-project.rb

class Project < ActiveRecord::Base
  # Add multiple images
  has_many :images, :dependent => :destroy
  accepts_nested_attributes_for :images, :allow_destroy => true
  # don't forget allow_destroy to be able to delete images through Project
end

If you have an error new_record? method does not exists, you probably forgot to add accepts_nested_attributes_for in your model.