_new_activity.html.erb

<%= form_for [@company, @activity], :remote=> true, :validate=>true do |f| %>

<%= f.label :title %>
<%= f.text_field :title, :class => "input-width bottom-border"%>

<%= f.label :activity_category_id %>
<%= f.collection_select(:activity_category_id, ActivityCategory.all, :id, :activity_category) %>

<%= f.label :background %>
<%= f.text_area :background, :class => "area-height area-width bottom-border" %>

<%= f.submit "Add activity", class: "btn submit-deals btn-info" %>
<% end %>

activities_controller.rb

class ActivitiesController < ApplicationController
 
  def create
  
  @company = Company.find(params[:company_id])
  @activity = @company.activities.build(params[:activity])
  flash[:success] = 'Activity Created!'
  respond_to do |format|
  	if @activity.save
  		format.html
  		format.js
  	else
  		render :json => { }, status => 500
  end

end
end
end 

activity.rb

class Activity < ActiveRecord::Base
  attr_accessible :activity_category_id, :background, :title, :user_id
  belongs_to :company
  belongs_to :contact
  belongs_to :activity_category
end

activity_category.rb

class ActivityCategory < ActiveRecord::Base
  attr_accessible :activity_category
  has_many :activities

end

console

  Activity Load (0.1ms)  SELECT "activities".* FROM "activities" ORDER BY "activities"."id" DESC LIMIT 1
 => #<Activity id: 3, title: "activity", background: "aa", activity_category_id: 2, user_id: nil, created_at: "2013-09-04 09:03:34", updated_at: "2013-09-04 09:03:34", company_id: 1, contact_id: nil> 
1.9.3p448 :002 > ActivityCategory.last

  ActivityCategory Load (0.3ms)  SELECT "activity_categories".* FROM "activity_categories" ORDER BY "activity_categories"."id" DESC LIMIT 1
 => #<ActivityCategory id: 3, activity_category: "Phone Call", created_at: "2013-09-04 08:49:54", updated_at: "2013-09-04 08:49:54"> 
1.9.3p448 :003 > 

log

Started POST "/companies/1/activities" for 127.0.0.1 at 2013-09-04 12:03:34 +0300
Processing by ActivitiesController#create as JS
  Parameters: {"utf8"=>"✓", "authenticity_token"=>"Nhflt/o9HdfsWXpsSaKddd5yvItOsGtDoFD5N/Rbfns=", "activity"=>{"title"=>"activity", "activity_category_id"=>"2", "background"=>"aa"}, "commit"=>"Add activity", "company_id"=>"1"}
  Company Load (0.1ms)  SELECT "companies".* FROM "companies" WHERE "companies"."id" = ? LIMIT 1  [["id", "1"]]
   (0.1ms)  begin transaction
  SQL (0.4ms)  INSERT INTO "activities" ("activity_category_id", "background", "company_id", "contact_id", "created_at", "title", "updated_at", "user_id") VALUES (?, ?, ?, ?, ?, ?, ?, ?)  [["activity_category_id", 2], ["background", "aa"], ["company_id", 1], ["contact_id", nil], ["created_at", Wed, 04 Sep 2013 09:03:34 UTC +00:00], ["title", "activity"], ["updated_at", Wed, 04 Sep 2013 09:03:34 UTC +00:00], ["user_id", nil]]
   (201.5ms)  commit transaction
  Rendered activities/_activity.html.erb (0.0ms)
  Rendered activities/create.js.erb (0.7ms)
Completed 200 OK in 212ms (Views: 2.9ms | ActiveRecord: 202.1ms)

seeds.rb

["Meeting", "Email", "Phone Call"].each do |category|
ActivityCategory.create(:activity_category => category)
end