adding papertrail migrations

db/migrate/20160512140322_ensure_versions_table_is_deleted.rb

+class EnsureVersionsTableIsDeleted < ActiveRecord::Migration
+  def change
+    table = :versions
+    drop_table table if table_exists? table
+  end
+end

db/migrate/20160512143532_create_versions.rb

+# This migration creates the `versions` table, the only schema PT requires.
+# All other migrations PT provides are optional.
+class CreateVersions < ActiveRecord::Migration
+  # Class names of MySQL adapters.
+  # - `MysqlAdapter` - Used by gems: `mysql`, `activerecord-jdbcmysql-adapter`.
+  # - `Mysql2Adapter` - Used by `mysql2` gem.
+  MYSQL_ADAPTERS = [
+    "ActiveRecord::ConnectionAdapters::MysqlAdapter",
+    "ActiveRecord::ConnectionAdapters::Mysql2Adapter"
+  ].freeze
+
+  # The largest text column available in all supported RDBMS is
+  # 1024^3 - 1 bytes, roughly one gibibyte.  We specify a size
+  # so that MySQL will use `longtext` instead of `text`.  Otherwise,
+  # when serializing very large objects, `text` might not be big enough.
+  TEXT_BYTES = 1_073_741_823
+
+  def change
+    create_table :versions, versions_table_options do |t|
+      t.string   :item_type, null: false
+      t.integer  :item_id,   null: false
+      t.string   :event,     null: false
+      t.string   :whodunnit
+      t.text     :object, limit: TEXT_BYTES
+
+      # Known issue in MySQL: fractional second precision
+      # -------------------------------------------------
+      #
+      # MySQL timestamp columns do not support fractional seconds unless
+      # defined with "fractional seconds precision". MySQL users should manually
+      # add fractional seconds precision to this migration, specifically, to
+      # the `created_at` column.
+      # (https://dev.mysql.com/doc/refman/5.6/en/fractional-seconds.html)
+      #
+      # MySQL users should also upgrade to rails 4.2, which is the first
+      # version of ActiveRecord with support for fractional seconds in MySQL.
+      # (https://github.com/rails/rails/pull/14359)
+      #
+      t.datetime :created_at
+    end
+    add_index :versions, [:item_type, :item_id]
+  end
+
+  private
+
+  # Even modern versions of MySQL still use `latin1` as the default character
+  # encoding. Many users are not aware of this, and run into trouble when they
+  # try to use PaperTrail in apps that otherwise tend to use UTF-8. Postgres, by
+  # comparison, uses UTF-8 except in the unusual case where the OS is configured
+  # with a custom locale.
+  #
+  # - https://dev.mysql.com/doc/refman/5.7/en/charset-applications.html
+  # - http://www.postgresql.org/docs/9.4/static/multibyte.html
+  #
+  # Furthermore, MySQL's original implementation of UTF-8 was flawed, and had
+  # to be fixed later by introducing a new charset, `utf8mb4`.
+  #
+  # - https://mathiasbynens.be/notes/mysql-utf8mb4
+  # - https://dev.mysql.com/doc/refman/5.5/en/charset-unicode-utf8mb4.html
+  #
+  def versions_table_options
+    if MYSQL_ADAPTERS.include?(connection.class.name)
+      { options: "ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_general_ci" }
+    else
+      {}
+    end
+  end
+end