fix mysql '0000-00-00 00:00:00' default on hats.doffed_at

Fixes #451

app/models/user.rb

@@ -31,9 +31,7 @@ class User < ActiveRecord::Base
     :through => :votes,
     :source => :story
   has_many :hats
-  has_many :wearable_hats,
-    -> { where('hats.doffed_at' => nil) },
-    :class_name => "Hat"
+  has_many :wearable_hats, -> { where('doffed_at is null') }, :class_name => "Hat"
 
   has_secure_password
 

db/migrate/20180131203555_fix_doffed_at.rb

@@ -0,0 +1,6 @@
+class FixDoffedAt < ActiveRecord::Migration[5.1]
+  def change
+    change_column_default :hats, :doffed_at, nil
+    Hat.where("doffed_at = ?", "0000-00-00 00:00:00").update_all(:doffed_at => nil)
+  end
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 20180131170318) do
+ActiveRecord::Schema.define(version: 20180131203555) do
 
   create_table "comments", id: :integer, unsigned: true, force: :cascade, options: "ENGINE=InnoDB DEFAULT CHARSET=utf8mb4" do |t|
     t.datetime "created_at", null: false