3 files changed, 142 insertions, 0 deletions
diff --git a/app/models/concerns/ignorable_column.rb b/app/models/concerns/ignorable_column.rb
new file mode 100644
index 00000000000..eb9f3423e48
--- /dev/null
+++ b/app/models/concerns/ignorable_column.rb
@@ -0,0 +1,28 @@
+# Module that can be included into a model to make it easier to ignore database
+# columns.
+#
+# Example:
+#
+#     class User < ActiveRecord::Base
+#       include IgnorableColumn
+#
+#       ignore_column :updated_at
+#     end
+#
+module IgnorableColumn
+  extend ActiveSupport::Concern
+
+  module ClassMethods
+    def columns
+      super.reject { |column| ignored_columns.include?(column.name) }
+    end
+
+    def ignored_columns
+      @ignored_columns ||= Set.new
+    end
+
+    def ignore_column(name)
+      ignored_columns << name.to_s
+    end
+  end
+end
diff --git a/app/models/concerns/renameable_column.rb b/app/models/concerns/renameable_column.rb
new file mode 100644
index 00000000000..3f905520fe9
--- /dev/null
+++ b/app/models/concerns/renameable_column.rb
@@ -0,0 +1,107 @@
+module RenameableColumn
+  extend ActiveSupport::Concern
+
+  module ClassMethods
+    def rename_column(old_column, new_column, migrations:)
+      unless migration_exists?(migrations[:add_new])
+        raise "Cannot find migration '#{migrations[:add_new]}'"
+      end
+
+      unless migration_exists?(migrations[:migrate_data])
+        raise "Cannot find migration '#{migrations[:migrate_data]}'"
+      end
+
+      unless migration_exists?(migrations[:remove_old])
+        raise "Cannot find migration '#{migrations[:remove_old]}'"
+      end
+
+      if migration_ran?(migrations[:remove_old])
+        old_column_removed(old_column, new_column)
+      elsif migration_ran?(migrations[:migrate_data])
+        data_migrated_from_old_to_new_column(old_column, new_column)
+      elsif migration_ran?(migrations[:add_new])
+        new_column_added(old_column, new_column)
+      end
+    end
+
+    private
+
+    def new_column_added(old_column, new_column)
+      log_column_rename_status(old_column, new_column)
+      Rails.logger.info "The `#{new_column}` column has been added, but the data has not yet been migrated, and the `#{old_column}` column has not yet been removed."
+      log_column_usage_instructions(old_column, new_column)
+
+      before_save do
+        self[new_column] = self[old_column]
+
+        true
+      end
+
+      define_method "#{new_column}=" do |new_value|
+        raise "Use `#{self.class.name}##{old_column}=` until data is migrated from `#{old_column}` to `#{new_column}`"
+      end
+
+      define_singleton_method column_name_method(new_column) do
+        old_column
+      end
+    end
+
+    def data_migrated_from_old_to_new_column(old_column, new_column)
+      log_column_rename_status(old_column, new_column)
+      Rails.logger.info "The `#{new_column}` column has been added and the data has been migrated, but the `#{old_column}` column has not yet been removed."
+      log_column_usage_instructions(old_column, new_column)
+
+      # We read and write to and from `new_column`, but the code still says `old_column`
+      include IgnorableColumn
+
+      ignore_column old_column
+
+      alias_attribute old_column, new_column
+
+      define_singleton_method column_name_method(new_column) do
+        new_column
+      end
+    end
+
+    def old_column_removed(old_column, new_column)
+      warn "WARNING: `#{self.name}` column `#{old_column}` has been renamed to `#{new_column}`."
+      warn "All code should be updated to use `#{new_column}` where `#{old_column}` or the value of `#{self.name}.#{column_name_method(new_column)}` is currently used, and `rename_column #{old_column.inspect}, #{new_column.inspect}` should be removed from `#{self.name}`."
+
+      alias_attribute old_column, new_column
+
+      define_singleton_method column_name_method(new_column) do
+        new_column
+      end
+    end
+
+    def log_column_rename_status(old_column, new_column)
+      Rails.logger.info "`#{self.name}` column `#{old_column}` is in the process of being renamed to `#{new_column}`."
+    end
+
+    def log_column_usage_instructions(old_column, new_column)
+      Rails.logger.info "Until the rename is complete, all code should continue to read and write from `#{old_column}`, which may be the actual attribute or an ActiveRecord alias. However, plain SQL and Arel queries should use `#{self.name}.#{column_name_method(new_column)}`, which will return `#{old_column.inspect}` or `#{new_column.inspect}` based on the current state of the database."
+    end
+
+    def column_name_method(new_column)
+      "#{new_column}_column_name"
+    end
+
+    def all_migrations
+      @all_migrations ||= Set.new(ActiveRecord::Migrator.migrations(['db/release_migrations']).map(&:version))
+    end
+
+    def migration_exists?(name)
+      version = name.match(/^([0-9]+)/)[1].to_i
+      all_migrations.include?(version)
+    end
+
+    def ran_migrations
+      @ran_migrations ||= Set.new(ActiveRecord::Migrator.get_all_versions)
+    end
+
+    def migration_ran?(name)
+      version = name.match(/^([0-9]+)/)[1].to_i
+      ran_migrations.include?(version)
+    end
+  end
+end
diff --git a/app/models/user.rb b/app/models/user.rb
index 39c1281179b..a3bec974f1d 100644
--- a/app/models/user.rb
+++ b/app/models/user.rb
@@ -1,6 +1,13 @@
 require 'carrierwave/orm/activerecord'
 
 class User < ActiveRecord::Base
+  include RenameableColumn
+  rename_column :username, :handle, migrations: {
+    add_new:      '20170316184328_add_handle_column_to_users',
+    migrate_data: '20170316190016_migrate_users_username_to_handle',
+    remove_old:   '20170316190051_remove_username_column_from_users'
+  }
+
   extend Gitlab::ConfigHelper
 
   include Gitlab::ConfigHelper