Simple has_many :through

_form.html.erb

<%= form_for @photo, :html => { :multipart => true } do |photo| %>

<div class="field">
  <%= photo.label :title %>
  <%= photo.text_field :title %>
</div>
<div class="field">
  <%= photo.file_field :image %>
</div>
<%= photo.fields_for :tags do |tag| %>
  <div class="field">
    <%= tag.label :key %>
    <%= tag.text_field :key %>
  </div>
  <div class="field">
    <%= tag.label :value %>
    <%= tag.text_field :value %>
  </div>
<% end %>

<div class="actions">
  <%= photo.submit %>
</div>
<% end %>

photo.rb

class Photo < ActiveRecord::Base
  belongs_to :user
  
  has_many :tag_photo_relationships, dependent: :destroy
  has_many :tags, :through => :tag_photo_relationships
  
  accepts_nested_attributes_for :tags

  has_attached_file :image, :styles => { 
  	:medium => "300x300>", :thumb => "100x100>" }
  validates_attachment :image, :attachment_presence => true
  # Validate content type
  validates_attachment_content_type :image, :content_type => /\Aimage\/.*\Z/
  # Validate filename
  validates_attachment_file_name :image, :matches => [/png\Z/, /jpe?g\Z/]
end

photos_controller.rb

class PhotosController < ApplicationController
  before_action :set_photo, only: [:show]
  before_action :correct_user, only: [ :edit, :update, :destroy ]
  before_action :authenticate_user!


  # GET /photos
  # GET /photos.json
  def index
    @photos = current_user.photos
  end

  # GET /photos/1
  # GET /photos/1.json
  def show
    @tags = @photo.tags
  end

  # GET /photos/new
  def new
    @photo = current_user.photos.build
    @tag = @photo.tags.build
  end

  # GET /photos/1/edit
  def edit
  end

  # POST /photos
  # POST /photos.json
  def create
    @photo = current_user.photos.create(photo_params)

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

  # PATCH/PUT /photos/1
  # PATCH/PUT /photos/1.json
  def update
    respond_to do |format|
      if @photo.update(edit_photo_params)
        format.html { redirect_to @photo, notice: 'Photo was successfully updated.' }
        format.json { render :show, status: :ok, location: @photo }
      else
        format.html { render :edit }
        format.json { render json: @photo.errors, status: :unprocessable_entity }
      end
    end
  end

  # DELETE /photos/1
  # DELETE /photos/1.json
  def destroy
    @photo.image = nil
    @photo.save
    @photo.destroy
    respond_to do |format|
      format.html { redirect_to photos_url, notice: 'Photo was successfully destroyed.' }
      format.json { head :no_content }
    end
  end

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

    def correct_user
      @photo = current_user.photos.find_by(id: params[:id])
    end

    # Never trust parameters from the scary internet, only allow the white list through.
    def photo_params
      params.require(:photo).permit(:title, :image, :tags_attributes => [:id, :key, :value])
    end

    def edit_photo_params
      params.require(:photo).permit(:title)

    end
end

show.html.erb

<div class="panel row">
<p id="notice"><%= notice %></p>

<p>
  <strong>Title:</strong>
  <%= @photo.title %>
</p>

<%= image_tag (@photo.image.url(:medium)) %>

<p>
  <strong>User:</strong>
  <%= @photo.user.username %>
</p>

<%= link_to 'Edit', edit_photo_path(@photo), :class => "button radius round tiny" %> | 
<%= link_to 'Destroy', @photo, method: :delete, data: { confirm: 'Are you sure?' }, :class => "button tiny alert radius round tiny" %> | 
<%= link_to 'Back', photos_path, :class => "button [succes tiny radius round ]" %>

<h3>Tags</h3>
<%  @tags.each do |tag| %>
	<%=  tag.key %>
<% end %>


</div>

tag.rb

class Tag < ActiveRecord::Base
	has_many :tag_photo_relationships
	has_many :photos, :through => :tag_photo_relationships


	validates :key, presence: true, length: { in: 5..25 }, uniqueness: { case_sensitive: false }
	validates :value, presence: true, length: { in: 5..25 }
end

tag_photo_relationship.rb

class TagPhotoRelationship < ActiveRecord::Base
  belongs_to :tag
  belongs_to :photo
end

user.rb

class User < ActiveRecord::Base
  # Include default devise modules. Others available are:
  # :confirmable, :lockable, :timeoutable and :omniauthable
  devise :database_authenticatable, :registerable,
         :recoverable, :rememberable, :trackable, :validatable
  has_attached_file :avatar, :styles => { 
  	:medium => "300x300>", :thumb => "100x100>" }
  has_many :photos, :dependent => :destroy
  #validates :avatar, :attachment_presence => true
  #validates_with AttachmentSizeValidator, :attributes => :avatar, :less_than => 1.megabytes
  # Validate content type
  validates_attachment_content_type :avatar, :content_type => /\Aimage\/.*\Z/
  # Validate filename
  validates_attachment_file_name :avatar, :matches => [/png\Z/, /jpe?g\Z/]
end