Add LabelPriority model

app/models/label.rb

@@ -16,6 +16,7 @@ class Label < ActiveRecord::Base
   default_value_for :color, DEFAULT_COLOR
 
   has_many :lists, dependent: :destroy
+  has_many :priorities, class_name: 'LabelPriority'
   has_many :label_links, dependent: :destroy
   has_many :issues, through: :label_links, source: :target, source_type: 'Issue'
   has_many :merge_requests, through: :label_links, source: :target, source_type: 'MergeRequest'
@@ -26,8 +27,6 @@ class Label < ActiveRecord::Base
   validates :title, presence: true, format: { with: /\A[^,]+\z/ }
   validates :title, uniqueness: { scope: [:group_id, :project_id] }
 
-  before_save :nullify_priority
-
   default_scope { order(title: :asc) }
 
   scope :templates, -> { where(template: true) }
@@ -149,10 +148,6 @@ def label_format_reference(format = :id)
     end
   end
 
-  def nullify_priority
-    self.priority = nil if priority.blank?
-  end
-
   def sanitize_title(value)
     CGI.unescapeHTML(Sanitize.clean(value.to_s))
   end

app/models/label_priority.rb

+class LabelPriority < ActiveRecord::Base
+  belongs_to :project
+  belongs_to :label
+
+  validates :project, :label, :priority, presence: true
+  validates :label_id, uniqueness: { scope: :project_id }
+  validates :priority, numericality: { only_integer: true, greater_than_or_equal_to: 0 }
+end

db/migrate/20161014173530_create_label_priorities.rb

+class CreateLabelPriorities < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = true
+  DOWNTIME_REASON = 'Prioritezed labels will not work as expected until this migration is complete.'
+
+  disable_ddl_transaction!
+
+  def up
+    create_table :label_priorities do |t|
+      t.references :project, foreign_key: { on_delete: :cascade }, null: false
+      t.references :label, foreign_key: { on_delete: :cascade }, null: false
+      t.integer :priority, null: false
+
+      t.timestamps null: false
+    end
+
+    execute <<-EOF.strip_heredoc
+      INSERT INTO label_priorities (project_id, label_id, priority, created_at, updated_at)
+      SELECT labels.project_id, labels.id, labels.priority, NOW(), NOW()
+      FROM labels
+      WHERE labels.project_id IS NOT NULL
+        AND labels.priority IS NOT NULL;
+    EOF
+
+    add_concurrent_index :label_priorities, [:project_id, :label_id], unique: true
+    add_concurrent_index :label_priorities, :priority
+
+    remove_column :labels, :priority
+  end
+
+  def down
+    add_column :labels, :priority, :integer
+
+    if Gitlab::Database.mysql?
+      execute <<-EOF.strip_heredoc
+        UPDATE labels
+          INNER JOIN label_priorities ON labels.id = label_priorities.label_id AND labels.project_id = label_priorities.project_id
+        SET labels.priority = label_priorities.priority;
+      EOF
+    else
+      execute <<-EOF.strip_heredoc
+        UPDATE labels
+        SET priority = label_priorities.priority
+        FROM label_priorities
+        WHERE labels.id = label_priorities.label_id
+          AND labels.project_id = label_priorities.project_id;
+      EOF
+    end
+
+    add_concurrent_index :labels, :priority
+
+    drop_table :label_priorities
+  end
+end

db/schema.rb

@@ -519,6 +519,17 @@
   add_index "label_links", ["label_id"], name: "index_label_links_on_label_id", using: :btree
   add_index "label_links", ["target_id", "target_type"], name: "index_label_links_on_target_id_and_target_type", using: :btree
 
+  create_table "label_priorities", force: :cascade do |t|
+    t.integer "project_id", null: false
+    t.integer "label_id", null: false
+    t.integer "priority", null: false
+    t.datetime "created_at", null: false
+    t.datetime "updated_at", null: false
+  end
+
+  add_index "label_priorities", ["priority"], name: "index_label_priorities_on_priority", using: :btree
+  add_index "label_priorities", ["project_id", "label_id"], name: "index_label_priorities_on_project_id_and_label_id", unique: true, using: :btree
+
   create_table "labels", force: :cascade do |t|
     t.string "title"
     t.string "color"
@@ -527,14 +538,12 @@
     t.datetime "updated_at"
     t.boolean "template", default: false
     t.string "description"
-    t.integer "priority"
     t.text "description_html"
     t.string "type"
     t.integer "group_id"
   end
 
   add_index "labels", ["group_id"], name: "index_labels_on_group_id", using: :btree
-  add_index "labels", ["priority"], name: "index_labels_on_priority", using: :btree
   add_index "labels", ["project_id"], name: "index_labels_on_project_id", using: :btree
   add_index "labels", ["title"], name: "index_labels_on_title", using: :btree
 
@@ -1216,6 +1225,8 @@
 
   add_foreign_key "boards", "projects"
   add_foreign_key "issue_metrics", "issues", on_delete: :cascade
+  add_foreign_key "label_priorities", "labels", on_delete: :cascade
+  add_foreign_key "label_priorities", "projects", on_delete: :cascade
   add_foreign_key "labels", "namespaces", column: "group_id", on_delete: :cascade
   add_foreign_key "lists", "boards"
   add_foreign_key "lists", "labels"

spec/factories/label_priorities.rb

+FactoryGirl.define do
+  factory :label_priority do
+    project factory: :empty_project
+    label
+    sequence(:priority)
+  end
+end

spec/models/label_priority_spec.rb

+require 'spec_helper'
+
+describe LabelPriority, models: true do
+  describe 'relationships' do
+    it { is_expected.to belong_to(:project) }
+    it { is_expected.to belong_to(:label) }
+  end
+
+  describe 'validations' do
+    it { is_expected.to validate_presence_of(:project) }
+    it { is_expected.to validate_presence_of(:label) }
+    it { is_expected.to validate_presence_of(:priority) }
+    it { is_expected.to validate_numericality_of(:priority).only_integer.is_greater_than_or_equal_to(0) }
+
+    it 'validates uniqueness of label_id scoped to project_id' do
+      create(:label_priority)
+
+      expect(subject).to validate_uniqueness_of(:label_id).scoped_to(:project_id)
+    end
+  end
+end

spec/models/label_spec.rb

@@ -10,6 +10,7 @@
     it { is_expected.to have_many(:issues).through(:label_links).source(:target) }
     it { is_expected.to have_many(:label_links).dependent(:destroy) }
     it { is_expected.to have_many(:lists).dependent(:destroy) }
+    it { is_expected.to have_many(:priorities).class_name('LabelPriority') }
   end
 
   describe 'validation' do