Database table for language trends over time

- Introduce new DB table for tracking language trend over time.
- Initial model for Analytics::LanguageTrend::RepositoryLanguage

changelogs/unreleased/language-trends-over-time.yml

+---
+title: Database table for tracking programming language trends over time
+merge_request: 16491
+author:
+type: added

db/migrate/20190910125852_create_analytics_language_trend_repository_languages.rb

+# frozen_string_literal: true
+
+class CreateAnalyticsLanguageTrendRepositoryLanguages < ActiveRecord::Migration[5.2]
+  DOWNTIME = false
+  INDEX_PREFIX = 'analytics_repository_languages_'
+
+  def change
+    create_table :analytics_language_trend_repository_languages, id: false do |t|
+      t.integer :file_count, null: false, default: 0
+      t.references :programming_language, {
+        null: false,
+        foreign_key: { on_delete: :cascade },
+        index: false
+      }
+      t.references :project, {
+        null: false,
+        foreign_key: { on_delete: :cascade },
+        index: { name: INDEX_PREFIX + 'on_project_id' }
+      }
+      t.integer :loc, null: false, default: 0
+      t.integer :bytes, null: false, default: 0
+      # Storing percentage (with 2 decimal places), on 2 bytes.
+      # 50.25% => 5025
+      # Max: 100.00% => 10000 (fits smallint: 32767)
+      t.integer :percentage, limit: 2, null: false, default: 0
+      t.date :snapshot_date, null: false
+    end
+
+    add_index :analytics_language_trend_repository_languages, %I[
+      programming_language_id
+      project_id
+      snapshot_date
+    ], name: INDEX_PREFIX + 'unique_index', unique: true
+  end
+end

db/schema.rb

@@ -81,6 +81,18 @@ ActiveRecord::Schema.define(version: 2019_09_12_061145) do
     t.index ["start_event_label_id"], name: "index_analytics_ca_project_stages_on_start_event_label_id"
   end
 
+  create_table "analytics_language_trend_repository_languages", id: false, force: :cascade do |t|
+    t.integer "file_count", default: 0, null: false
+    t.bigint "programming_language_id", null: false
+    t.bigint "project_id", null: false
+    t.integer "loc", default: 0, null: false
+    t.integer "bytes", default: 0, null: false
+    t.integer "percentage", limit: 2, default: 0, null: false
+    t.date "snapshot_date", null: false
+    t.index ["programming_language_id", "project_id", "snapshot_date"], name: "analytics_repository_languages_unique_index", unique: true
+    t.index ["project_id"], name: "analytics_repository_languages_on_project_id"
+  end
+
   create_table "appearances", id: :serial, force: :cascade do |t|
     t.string "title", null: false
     t.text "description", null: false
@@ -3762,6 +3774,8 @@ ActiveRecord::Schema.define(version: 2019_09_12_061145) do
   add_foreign_key "analytics_cycle_analytics_project_stages", "labels", column: "end_event_label_id", on_delete: :cascade
   add_foreign_key "analytics_cycle_analytics_project_stages", "labels", column: "start_event_label_id", on_delete: :cascade
   add_foreign_key "analytics_cycle_analytics_project_stages", "projects", on_delete: :cascade
+  add_foreign_key "analytics_language_trend_repository_languages", "programming_languages", on_delete: :cascade
+  add_foreign_key "analytics_language_trend_repository_languages", "projects", on_delete: :cascade
   add_foreign_key "application_settings", "namespaces", column: "custom_project_templates_group_id", on_delete: :nullify
   add_foreign_key "application_settings", "projects", column: "file_template_project_id", name: "fk_ec757bd087", on_delete: :nullify
   add_foreign_key "application_settings", "projects", column: "instance_administration_project_id", on_delete: :nullify

ee/app/models/analytics/language_trend.rb

+# frozen_string_literal: true
+
+module Analytics::LanguageTrend
+  def self.table_name_prefix
+    'analytics_language_trend_'
+  end
+end

ee/app/models/analytics/language_trend/repository_language.rb

+# frozen_string_literal: true
+
+class Analytics::LanguageTrend::RepositoryLanguage < ApplicationRecord
+  belongs_to :programming_language
+  belongs_to :project
+end

ee/spec/models/analytics/language_trend/repository_language_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe Analytics::LanguageTrend::RepositoryLanguage do
+  describe 'associations' do
+    it { is_expected.to belong_to(:programming_language) }
+    it { is_expected.to belong_to(:project) }
+  end
+end