Use normal associations instead of polymorphic.

We can't properly use foreign keys on columns that are configured for
polymorphic associations which has disadvantages related to data
integrity and storage. Given we only use time tracking for Issues and
Merge Requests we're moving to the usage of regular associations.

app/models/concerns/time_trackable.rb

@@ -18,7 +18,7 @@ module TimeTrackable
     validates :time_estimate, numericality: { message: 'has an invalid format' }, allow_nil: false
     validate  :check_negative_time_spent
 
-    has_many :timelogs, as: :trackable, dependent: :destroy
+    has_many :timelogs, dependent: :destroy
   end
 
   def spend_time(options)

app/models/timelog.rb

 class Timelog < ActiveRecord::Base
   validates :time_spent, :user, presence: true
+  validate :issuable_id_is_present
 
-  belongs_to :trackable, polymorphic: true
+  belongs_to :issue
+  belongs_to :merge_request
   belongs_to :user
+
+  def issuable
+    issue || merge_request
+  end
+
+  private
+
+  def issuable_id_is_present
+    if issue_id && merge_request_id
+      errors.add(:base, 'Only Issue ID or Merge Request ID is required')
+    elsif issuable.nil?
+      errors.add(:base, 'Issue or Merge Request ID is required')
+    end
+  end
 end

changelogs/unreleased/26908-make-timelogs-use-foreign-keys

+---
+title: Refactor Timelogs structure to use foreign keys.
+merge_request: 8769
+author:

db/migrate/20170124174637_add_foreign_keys_to_timelogs.rb

+# See http://doc.gitlab.com/ce/development/migration_style_guide.html
+# for more information on how to write migrations for GitLab.
+
+class AddForeignKeysToTimelogs < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  # Set this constant to true if this migration requires downtime.
+  DOWNTIME = true
+  # When a migration requires downtime you **must** uncomment the following
+  # constant and define a short and easy to understand explanation as to why the
+  # migration requires downtime.
+  DOWNTIME_REASON = 'Adding foreign keys'
+
+  # When using the methods "add_concurrent_index" or "add_column_with_default"
+  # you must disable the use of transactions as these methods can not run in an
+  # existing transaction. When using "add_concurrent_index" make sure that this
+  # method is the _only_ method called in the migration, any other changes
+  # should go in a separate migration. This ensures that upon failure _only_ the
+  # index creation fails and can be retried or reverted easily.
+  #
+  # To disable transactions uncomment the following line and remove these
+  # comments:
+  # disable_ddl_transaction!
+
+  def up
+    change_table :timelogs do |t|
+      t.references :issue, index: true, foreign_key: { on_delete: :cascade }
+      t.references :merge_request, index: true, foreign_key: { on_delete: :cascade }
+    end
+
+    Timelog.where(trackable_type: 'Issue').update_all("issue_id = trackable_id")
+    Timelog.where(trackable_type: 'MergeRequest').update_all("merge_request_id = trackable_id")
+
+    remove_columns :timelogs, :trackable_id, :trackable_type
+  end
+
+  def down
+    add_reference :timelogs, :trackable, polymorphic: true, index: true
+
+    Timelog.where('issue_id IS NOT NULL').update_all("trackable_id = issue_id, trackable_type = 'Issue'")
+    Timelog.where('merge_request_id IS NOT NULL').update_all("trackable_id = merge_request_id, trackable_type = 'MergeRequest'")
+
+    remove_columns :timelogs, :issue_id, :merge_request_id
+  end
+end

db/schema.rb

@@ -1151,14 +1151,15 @@ ActiveRecord::Schema.define(version: 20170206071414) do
 
   create_table "timelogs", force: :cascade do |t|
     t.integer "time_spent", null: false
-    t.integer "trackable_id"
-    t.string "trackable_type"
     t.integer "user_id"
     t.datetime "created_at", null: false
     t.datetime "updated_at", null: false
+    t.integer "issue_id"
+    t.integer "merge_request_id"
   end
 
-  add_index "timelogs", ["trackable_type", "trackable_id"], name: "index_timelogs_on_trackable_type_and_trackable_id", using: :btree
+  add_index "timelogs", ["issue_id"], name: "index_timelogs_on_issue_id", using: :btree
+  add_index "timelogs", ["merge_request_id"], name: "index_timelogs_on_merge_request_id", using: :btree
   add_index "timelogs", ["user_id"], name: "index_timelogs_on_user_id", using: :btree
 
   create_table "todos", force: :cascade do |t|
@@ -1340,6 +1341,8 @@ ActiveRecord::Schema.define(version: 20170206071414) do
   add_foreign_key "protected_branch_merge_access_levels", "protected_branches"
   add_foreign_key "protected_branch_push_access_levels", "protected_branches"
   add_foreign_key "subscriptions", "projects", on_delete: :cascade
+  add_foreign_key "timelogs", "issues", on_delete: :cascade
+  add_foreign_key "timelogs", "merge_requests", on_delete: :cascade
   add_foreign_key "trending_projects", "projects", on_delete: :cascade
   add_foreign_key "u2f_registrations", "users"
 end

spec/factories/timelogs.rb

@@ -4,6 +4,6 @@ FactoryGirl.define do
   factory :timelog do
     time_spent 3600
     user
-    association :trackable, factory: :issue
+    issue
   end
 end

spec/lib/gitlab/import_export/all_models.yml

@@ -203,5 +203,6 @@ award_emoji:
 priorities:
 - label
 timelogs:
-- trackable
+- issue
+- merge_request
 - user

spec/lib/gitlab/import_export/safe_model_attributes.yml

@@ -350,8 +350,8 @@ LabelPriority:
 Timelog:
 - id
 - time_spent
-- trackable_id
-- trackable_type
+- merge_request_id
+- issue_id
 - user_id
 - created_at
 - updated_at

spec/models/timelog_spec.rb

@@ -2,9 +2,37 @@ require 'rails_helper'
 
 RSpec.describe Timelog, type: :model do
   subject { build(:timelog) }
+  let(:issue) { create(:issue) }
+  let(:merge_request) { create(:merge_request) }
 
   it { is_expected.to be_valid }
 
   it { is_expected.to validate_presence_of(:time_spent) }
   it { is_expected.to validate_presence_of(:user) }
+
+  describe 'Issuable validation' do
+    it 'is invalid if issue_id and merge_request_id are missing' do
+      subject.attributes = { issue: nil, merge_request: nil }
+
+      expect(subject).to be_invalid
+    end
+
+    it 'is invalid if issue_id and merge_request_id are set' do
+      subject.attributes = { issue: issue, merge_request: merge_request }
+
+      expect(subject).to be_invalid
+    end
+
+    it 'is valid if only issue_id is set' do
+      subject.attributes = { issue: issue, merge_request: nil }
+
+      expect(subject).to be_valid
+    end
+
+    it 'is valid if only merge_request_id is set' do
+      subject.attributes = { merge_request: merge_request, issue: nil }
+
+      expect(subject).to be_valid
+    end
+  end
 end