Remove relative_position default value

Add migration to set existing child epics' positions

db/schema.rb

@@ -1114,7 +1114,7 @@ ActiveRecord::Schema.define(version: 20190220150130) do
     t.integer "closed_by_id"
     t.datetime "closed_at"
     t.integer "parent_id"
-    t.integer "relative_position", default: 1073741823, null: false
+    t.integer "relative_position"
     t.index ["assignee_id"], name: "index_epics_on_assignee_id", using: :btree
     t.index ["author_id"], name: "index_epics_on_author_id", using: :btree
     t.index ["closed_by_id"], name: "index_epics_on_closed_by_id", using: :btree

ee/db/migrate/20190128104236_add_relative_position_to_epics.rb

@@ -8,11 +8,8 @@ class AddRelativePositionToEpics < ActiveRecord::Migration[5.0]
 
   DOWNTIME = false
 
-  disable_ddl_transaction!
-
   def up
-    default_position = Gitlab::Database::MAX_INT_VALUE / 2
-    add_column_with_default(:epics, :relative_position, :integer, default: default_position, allow_null: false)
+    add_column :epics, :relative_position, :integer
   end
 
   def down

ee/db/migrate/20190218031401_set_default_position_for_child_epics.rb

+# frozen_string_literal: true
+
+# See http://doc.gitlab.com/ce/development/migration_style_guide.html
+# for more information on how to write migrations for GitLab.
+
+class SetDefaultPositionForChildEpics < ActiveRecord::Migration[5.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  def up
+    current_position = nil
+    current_parent_id = nil
+
+    connection.exec_query(existing_child_epics_query.to_sql).rows.each do |id, parent_id|
+      if parent_id != current_parent_id
+        current_position = Gitlab::Database::MAX_INT_VALUE / 2
+        current_parent_id = parent_id
+      else
+        current_position += 500
+      end
+
+      update_position(id, current_position)
+    end
+  end
+
+  def down
+  end
+
+  private
+
+  def epics_table
+    @epics_table ||= Arel::Table.new(:epics)
+  end
+
+  def existing_child_epics_query
+    epics_table.project(epics_table[:id], epics_table[:parent_id])
+      .where(epics_table[:parent_id].not_eq(nil))
+      .order(epics_table[:parent_id], epics_table[:id].desc)
+  end
+
+  def update_position(epic_id, position)
+    execute "UPDATE epics SET relative_position = #{position} WHERE id = #{epic_id}"
+  end
+end

ee/spec/services/epic_links/create_service_spec.rb

@@ -19,7 +19,7 @@ describe EpicLinks::CreateService, :postgresql do
       end
 
       it 'moves the new child epic to the top and moves the existing ones down' do
-        existing_child_epic = create(:epic, group: group, parent: epic)
+        existing_child_epic = create(:epic, group: group, parent: epic, relative_position: 1000)
 
         subject