Add date fixed/from_migrations columns to Epic

Indicate whether start and due dates should be sourced from Epic's issue's milestones.

Post migrate epics to copy milestone dates

db/schema.rb

@@ -954,6 +954,10 @@ ActiveRecord::Schema.define(version: 20180803001726) do
     t.string "title_html", null: false
     t.text "description"
     t.text "description_html"
+    t.boolean "start_date_is_fixed"
+    t.boolean "due_date_is_fixed"
+    t.date "start_date_fixed"
+    t.date "due_date_fixed"
   end
 
   add_index "epics", ["assignee_id"], name: "index_epics_on_assignee_id", using: :btree

ee/db/migrate/20180711014025_add_date_columns_to_epics.rb

+class AddDateColumnsToEpics < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  def change
+    change_table :epics do |t|
+      t.boolean :start_date_is_fixed
+      t.boolean :due_date_is_fixed
+      t.date :start_date_fixed
+      t.date :due_date_fixed
+    end
+  end
+end

ee/db/migrate/20180711014026_update_date_columns_on_epics.rb

+class UpdateDateColumnsOnEpics < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+  disable_ddl_transaction!
+
+  class Epic < ActiveRecord::Base
+    self.table_name = 'epics'
+    include EachBatch
+  end
+
+  def up
+    Epic.where.not(start_date: nil).each_batch do |batch|
+      batch.update_all('start_date_is_fixed = true, start_date_fixed = start_date')
+    end
+
+    Epic.where.not(end_date: nil).each_batch do |batch|
+      batch.update_all('due_date_is_fixed = true, due_date_fixed = end_date')
+    end
+  end
+
+  def down
+    # NOOP
+  end
+end

ee/db/post_migrate/20180713171825_update_epic_dates_from_milestones.rb

+class UpdateEpicDatesFromMilestones < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  class Epic < ActiveRecord::Base
+    self.table_name = 'epics'
+    include EachBatch
+  end
+
+  def up
+    Epic.where(start_date: nil).find_each do |epic|
+      epic.update_dates
+    end
+  end
+
+  def down
+    # NOOP
+  end
+end