Many to Many relationship.

gistfile1.txt

{"utf8"=>"✓",
 "authenticity_token"=>"3WtF72DqfVNI9PSGMdiR5ZgWrzPuZAp+89hV+nZp6eA=",
 "tag"=>{"key"=>"aoe",
 "value"=>"aoe"},
 "commit"=>"Tag Photo",
 "photo_id"=>"5"}

photos_controller.rb

class PhotosController < ApplicationController
  before_action :set_photo, only: [:show]
  before_action :correct_user, only: [ :show, :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
    @tag = Tag.new
  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, :_destroy])
    end

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

    end
end

photosshow.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>
<p>           
    <%= form_for([@photo, @tag]) do |f| %>
        <div class="field">
            <%= f.label :key %>
            <%= f.text_field :key %>
        </div>
        <div class="field">
            <%= f.label :value %>
            <%= f.text_field :value %>
        </div>
        <%= f.submit "Tag Photo", class: "button success" %>
    <% end %>
</p>
<%  @tags.each do |tag| %>
<p>Key: <%=  tag.key %></p>
<p>Value: <%=  tag.value %></p>
<% end %>




</div>

Redirection

   (0.1ms)  begin transaction
  SQL (2.4ms)  INSERT INTO "tags" ("created_at", "key", "updated_at", "value") VALUES (?, ?, ?, ?)  [["created_at", "2014-09-05 04
:02:04.349445"], ["key", "heydude"], ["updated_at", "2014-09-05 04:02:04.349445"], ["value", "dudehey"]]
   (4.3ms)  commit transaction

Reference

http://apidock.com/rails/ActionView/Helpers/FormHelper/form_for
If your resource has associations defined, for example, you want to add comments to the document given that the routes are set correctly:

<%= form_for([@document, @comment]) do |f| %>
 ...
<% end %>
Where @document = Document.find(params[:id]) and @comment = Comment.new.

tags_controller.rb

class TagsController < ApplicationController
	before_action :set_photo, only: [:create]

  def create
  	@tag = @photo.tags.build(tag_params)
  	if @tag.save
  		flash[:success] = "Photo Tagged"
  		redirect_to @photo
  	else
  		flash[:error] = "Failed To Tag"
  		redirect_to @photo
  	end
  end  

private
	def set_photo
		@photo = current_user.photos.find_by(id: params[:photo_id])
		redirect_to root_url if @photo.nil?
	end

	def tag_params
		params.require(:tag).permit(:id, :key, :value, :photo_id)
	end
end