Persist target project id and target branch in merge trains table

In order to optimize the query performance, we should use target
project id and target branch intead of cross-joining  merge_requests
table.

Fix schema

Fix schema

db/migrate/20190611161641_add_target_project_id_to_merge_trains.rb

+# frozen_string_literal: true
+
+class AddTargetProjectIdToMergeTrains < ActiveRecord::Migration[5.1]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  def change
+    # rubocop: disable Rails/NotNullColumn
+    add_reference :merge_trains, :target_project, null: false, index: true, foreign_key: { on_delete: :cascade, to_table: :projects }, type: :integer
+    add_column :merge_trains, :target_branch, :text, null: false
+    # rubocop: enable Rails/NotNullColumn
+  end
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 20190530154715) do
+ActiveRecord::Schema.define(version: 20190611161641) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -1387,8 +1387,11 @@ ActiveRecord::Schema.define(version: 20190530154715) do
     t.integer "pipeline_id"
     t.datetime_with_timezone "created_at", null: false
     t.datetime_with_timezone "updated_at", null: false
+    t.integer "target_project_id", null: false
+    t.text "target_branch", null: false
     t.index ["merge_request_id"], name: "index_merge_trains_on_merge_request_id", unique: true, using: :btree
     t.index ["pipeline_id"], name: "index_merge_trains_on_pipeline_id", using: :btree
+    t.index ["target_project_id"], name: "index_merge_trains_on_target_project_id", using: :btree
     t.index ["user_id"], name: "index_merge_trains_on_user_id", using: :btree
   end
 
@@ -2570,6 +2573,7 @@ ActiveRecord::Schema.define(version: 20190530154715) do
   add_foreign_key "merge_requests_closing_issues", "merge_requests", on_delete: :cascade
   add_foreign_key "merge_trains", "ci_pipelines", column: "pipeline_id", on_delete: :nullify
   add_foreign_key "merge_trains", "merge_requests", on_delete: :cascade
+  add_foreign_key "merge_trains", "projects", column: "target_project_id", on_delete: :cascade
   add_foreign_key "merge_trains", "users", on_delete: :cascade
   add_foreign_key "milestones", "namespaces", column: "group_id", name: "fk_95650a40d4", on_delete: :cascade
   add_foreign_key "milestones", "projects", name: "fk_9bd0a0c791", on_delete: :cascade