title: Factory girl object with has many through and presence validation
body: <p>I have a model named Beats which consists of fields like name, status, group, owner. I am writing tests using R Spec and mocking using Factory Girl, I am facing issue running specs it gives error as <strong>ActiveRecord::RecordInvalid: Validation failed: Beat Types can't be blank</strong>, I am validating presence of all the fields of  form, there is a dropdown with BeatType values, its also getting called when running specs, whats the way to include it in Factory of Beat? </p>
  
  <pre><code>class Beat &lt; ActiveRecord::Base
  has_many :beat_beat_types
  has_many :beat_types, through: :beat_beat_types
  
  validates :name,:status,:group,:owner,:beat_types presence: true, length: {maximum: 255}
  end
  
  class BeatType &lt; ActiveRecord::Base
  has_many :beat_beat_types
  has_many :beats, through: :beat_beat_types
  end
  
  class BeatBeatType &lt; ActiveRecord::Base
  belongs_to :beat
  belongs_to :beat_type
  end
  
  
  Factory_File of beat
  
  FactoryGirl.define do
  factory :beat do
    name
    status
    group
    owner
  end
  end
  </code></pre>