Recently i have used recaptcha gem in one of my rails application with bootstrap form controls. Here i am sharing my codes, so that someone can get help from this.

_form.html.haml

/ app/views/posts/_form.html.haml
= simple_form_for(@post) do |f|
  = f.error_notification

  .form-inputs
    = f.input :title
    = f.input :content, :as => :text
    = render 'shared/recaptcha'

  .form-actions
    = f.button :submit, 'Submit', :class => 'btn-primary'
    = link_to t('.cancel', :default => t("helpers.links.cancel")), :back, :class => 'btn'

_recaptcha.html.haml

/ /app/views/shared/_recaptcha.html.haml
#recaptcha_widget{:style => "display:none"}
  .control-group
    %label.control-label reCAPTCHA
    .controls
      %a#recaptcha_image.thumbnail{:href => "#"}
      %p.help-block
        To confirm that you are not a bot/computer please fill out the captcha below.
      .recaptcha_only_if_incorrect_sol{:style => "color:red"} Incorrect please try again
  .control-group
    %label.recaptcha_only_if_image.control-label * Enter the words above:
    %label.recaptcha_only_if_audio.control-label * Enter the numbers you hear:
    .controls
      .input-append
        %input#recaptcha_response_field.input-recaptcha{:name => "recaptcha_response_field", :type => "text"}/
        %a.btn{:href => "javascript:Recaptcha.reload()"}
          %i.icon-refresh
        %a.btn.recaptcha_only_if_image{:href => "javascript:Recaptcha.switch_type('audio')"}
          %i.icon-headphones{:title => "Get an audio CAPTCHA"}
        %a.btn.recaptcha_only_if_audio{:href => "javascript:Recaptcha.switch_type('image')"}
          %i.icon-picture{:title => "Get an image CAPTCHA"}
        %a.btn{:href => "javascript:Recaptcha.showhelp()"}
          %i.icon-question-sign
= recaptcha_tags :display => {:theme => 'custom', :custom_theme_widget => 'recaptcha_widget'}

posts_controller.rb

# app/controllers/posts_controller.rb
class PostsController < ApplicationController
  # GET /posts/new
  # GET /posts/new.json
  def new
    @posts = Post.new

    respond_to do |format|
      format.html # new.html.erb
      format.json { render json: @post }
    end
  end

  # POST /posts
  # POST /posts.json
  def create
    @post = Post.new(params[:post])
  
    respond_to do |format|
      if verify_recaptcha
        if @post.save
          format.html { redirect_to @post, notice: "Your post was successfully submitted" }
          format.json { render json: @post, status: :created, location: @post }
        else
          format.html { render action: "new" }
          format.json { render json: @post.errors, status: :unprocessable_entity }
        end
      else
        flash[:recaptcha_error] = 'Captcha is incorrect please try again.'
        format.html { render action: "new" }
        format.json { render json: @post.errors, status: :unprocessable_entity }
      end
    end
  end
end