Formtastic unknown input error - Unable to find input class for select

Question

In my application I have a Gallery class and a Project class, which are connected with a has_and_belongs_to_many relationship. I'm trying to add the ability to add projects to a gallery, using a dropdown select with all of the current projects in the database populated as options. My form for creating a new gallery is the following:

<%= semantic_form_for @gallery do |f| %>
  <%= f.inputs do %>
    <%= f.input :title %>
    <%= f.input :projects, :as => :select, :collection => Project.all,
                :include_blank => true, :input_html => { :multiple => true } %>
  <% end %>

  <%= f.actions do %>
    <%= f.action :submit, :as => :input %>
  <% end %>
<% end %>

However, when I go to the new gallery page, I get the following error

Formtastic::UnknownInputError in Galleries#new
Showing /app/views/galleries/_form.html.erb where line #4 raised:
Unable to find input class for select

I tried to follow the specifications as defined in the documentation, so why am I getting this error? Do I need to add something to my galleries_controller.rb?

Edit - Here are both of the models

class Gallery < ActiveRecord::Base
  has_and_belongs_to_many :projects
  belongs_to :user
  validates :title, uniqueness: true
  validates :title, length: { minimum: 4 }
  validates :user, presence: true
end

class Project < ActiveRecord::Base
  belongs_to :user
  has_many :comments
  has_and_belongs_to_many :galleries

  validates :title, presence: true
  validates :thumbnail, presence: true

  def root_comments
    comments.where parent_id: nil
  end
end

Answer 1

It turns out that this is a problem with gem versions between Formtastic and Formtastic-bootstrap. According to this issue:

formtastic-bootstrap 3.0 actually only support formtastic 2.x.

I was using Formtastic version 3.x so downgrading to Formtastic 2.x solved my problem. This is how I updated my Gemfile:

gem 'formtastic', '~> 2.0'