Add an `Issue::Metrics` model.

- And store the `first_associated_with_milestone_at` and
  `first_added_to_board_at` times, when an issue is saved.

app/models/issue.rb

@@ -23,6 +23,8 @@ class Issue < ActiveRecord::Base
 
   has_many :events, as: :target, dependent: :destroy
 
+  has_one :metrics, dependent: :destroy
+
   validates :project, presence: true
 
   scope :cared, ->(user) { where(assignee_id: user) }
@@ -36,6 +38,8 @@ class Issue < ActiveRecord::Base
   scope :order_due_date_asc, -> { reorder('issues.due_date IS NULL, issues.due_date ASC') }
   scope :order_due_date_desc, -> { reorder('issues.due_date IS NULL, issues.due_date DESC') }
 
+  after_save :record_metrics
+
   attr_spammable :title, spam_title: true
   attr_spammable :description, spam_description: true
 
@@ -270,4 +274,9 @@ class Issue < ActiveRecord::Base
   def check_for_spam?
     project.public?
   end
+
+  def record_metrics
+    metrics = Metrics.find_or_create_by(issue_id: self.id)
+    metrics.record!
+  end
 end

app/models/issue/metrics.rb

+class Issue::Metrics < ActiveRecord::Base
+  belongs_to :issue
+
+  def record!
+    if issue.milestone_id.present? && self.first_associated_with_milestone_at.blank?
+      self.first_associated_with_milestone_at = Time.now
+    end
+
+    if issue_assigned_to_list_label? && self.first_added_to_board_at.blank?
+      self.first_added_to_board_at = Time.now
+    end
+
+    self.save if self.changed?
+  end
+
+  private
+
+  def issue_assigned_to_list_label?
+    issue.labels.any? { |label| label.lists.present? }
+  end
+end

db/migrate/20160824124900_add_table_issue_metrics.rb

+# See http://doc.gitlab.com/ce/development/migration_style_guide.html
+# for more information on how to write migrations for GitLab.
+
+class AddTableIssueMetrics < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  # Set this constant to true if this migration requires downtime.
+  DOWNTIME = false
+
+  # 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 = ''
+
+  # 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 change
+    create_table :issue_metrics do |t|
+      t.references :issue, index: { name: "index_issue_metrics" }, foreign_key: true, null: false
+
+      t.datetime 'first_associated_with_milestone_at'
+      t.datetime 'first_added_to_board_at'
+
+      t.timestamps null: false
+    end
+  end
+end

db/schema.rb

@@ -11,7 +11,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 20160823081327) do
+ActiveRecord::Schema.define(version: 20160824124900) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -453,6 +453,16 @@ ActiveRecord::Schema.define(version: 20160823081327) do
 
   add_index "identities", ["user_id"], name: "index_identities_on_user_id", using: :btree
 
+  create_table "issue_metrics", force: :cascade do |t|
+    t.integer  "issue_id",                           null: false
+    t.datetime "first_associated_with_milestone_at"
+    t.datetime "first_added_to_board_at"
+    t.datetime "created_at",                         null: false
+    t.datetime "updated_at",                         null: false
+  end
+
+  add_index "issue_metrics", ["issue_id"], name: "index_issue_metrics", using: :btree
+
   create_table "issues", force: :cascade do |t|
     t.string   "title"
     t.integer  "assignee_id"
@@ -1150,6 +1160,7 @@ ActiveRecord::Schema.define(version: 20160823081327) do
   add_index "web_hooks", ["project_id"], name: "index_web_hooks_on_project_id", using: :btree
 
   add_foreign_key "boards", "projects"
+  add_foreign_key "issue_metrics", "issues"
   add_foreign_key "lists", "boards"
   add_foreign_key "lists", "labels"
   add_foreign_key "personal_access_tokens", "users"