controller.rb

class TasksController < ApplicationController
  def index
  
  end

  def show
    
  end

  def new
    @task = Task.new
    @new_task.due_date_word = 'Tomorrow'

  end

  def create
    @company = Company.find(params[:company_id])
    @task = @company.tasks.create(params[:task])

    if @task.save
      redirect_to "/tasks"
    else 
      render 'new'
    end
  end

  def edit
  end
end

new.html.erb

<div class="container">
<div class="content pull-left">
 <%= form_for(@task, :url => {:action => "create"}) do |f| %>

 <%= f.label :description %>
 <%= f.text_field :description, :class => "input-width bottom-border" %>
 
 <%= f.label :task_category_id, "Choose a category" %>
<%= f.collection_select(:task_category_id, TaskCategory.all, :id, :task_category) %>
<%= f.label :due_date %>
 <%= f.select :due_date_word, ['Today', 'Tomorrow', 'Next Week']%>
 <%= f.submit "Add this Task", class: "btn" %>
 <% end %>
</div>
</div>

tasks.rb

# == Schema Information
#
# Table name: tasks
#
#  id               :integer          not null, primary key
#  title            :text
#  task_category_id :integer
#  due_date         :datetime
#  description      :text
#  completed_at     :datetime
#  company_id       :integer
#  contact_id       :integer
#  user_id          :integer
#  created_at       :datetime         not null
#  updated_at       :datetime         not null
#

class Task < ActiveRecord::Base
  attr_accessible :company_id, :completed_at, :contact_id, :description, :due_date, :task_category_id, :title, :user_id, :due_date_word
  belongs_to :company
  belongs_to :task_category
  belongs_to :contact
  belongs_to :user
  before_save :check_due_date_word


  private
  def check_due_date_word
    case @due_date_word
    when 'Today' then self.due_date = Time.now
    when 'Tomorrow' then self.due_date = Time.now + 1.day
  end
end

class TasksController
  def new
    @task = Task.new
    @task.due_date_word = 'Tomorrow'
  end
end


class Task < ActiveRecord::Base
  attr_accessor :due_date_word
  before_save :check_due_date_word

  private
  def check_due_date_word
    case @due_date_word
    when 'Today' then self.due_date = Time.now
    when 'Tomorrow' then self.due_date = Time.now + 1.day
    end
  end
end