Maintain size of sponsor logos

Places logo on a white background if image is smaller than the given
constrains.
Resizes the logo if image is larger than constrains.
Validation of presence of logo was moved cause file field wasn't getting
marked as required field ('*' thingy) in form.
`- if logo presence` was removed cause logo is mandatory field.

app/helpers/sponsors_helper.rb

@@ -0,0 +1,12 @@
+module SponsorsHelper
+  # returns the url to be used for logo on basis of sponsorship level position
+  def get_logo(sponsor)
+    if sponsor.sponsorship_level.position == 1
+      sponsor.logo.url(:first)
+    elsif sponsor.sponsorship_level.position == 2
+      sponsor.logo.url(:second)
+    else
+      sponsor.logo.url(:others)
+    end
+  end
+end

app/models/sponsor.rb

@@ -2,12 +2,20 @@ class Sponsor < ActiveRecord::Base
   belongs_to :sponsorship_level
   belongs_to :conference
   has_attached_file :logo,
-                    styles: { thumb: '100x100>', large: '300x300>' }
+                    styles: { thumb: '100x100>',
+                              first: '320x180>',
+                              second: '320x150>',
+                              others: '320x120>' },
+                    # places logo on a white background to maintain size
+                    convert_options: {
+                      first: '-background white -gravity center -extent 320x180',
+                      second: '-background white -gravity center -extent 320x150',
+                      others: '-background white -gravity center -extent 320x120'
+                    }
 
   validates_attachment_content_type :logo,
                                     content_type: [/jpg/, /jpeg/, /png/, /gif/],
-                                    size: { in: 0..500.kilobytes },
-                                    presence: true
+                                    size: { in: 0..500.kilobytes }
 
-  validates_presence_of :name, :website_url, :sponsorship_level
+  validates_presence_of :name, :website_url, :sponsorship_level, :logo
 end

app/views/conference/_sponsors.html.haml

@@ -14,9 +14,8 @@
         .row
           - slice.each do |sponsor|
             .col-md-4.col-sm-4.col-top
-              - if sponsor.logo.present?
-                = link_to(sponsor.website_url, class: 'thumbnail') do
-                  = image_tag sponsor.logo, class: "img-responsive img-sponsor img-sponsor-#{sponsorship_level.position}", title: "#{sponsor.description}"
+              = link_to(sponsor.website_url, class: 'thumbnail') do
+                = image_tag get_logo(sponsor), class: "img-responsive img-sponsor img-sponsor-#{sponsorship_level.position}", title: "#{sponsor.description}"
   -if @conference.contact and !@conference.contact.sponsor_email.blank?
     .row
       .col-md-12