Add a foreign key to ci_variables to projects

Variables shouldn't exist without a project, so a foreign key was added
with CASCADE'ing effects.

db/migrate/20170508190732_add_foreign_key_to_ci_variables.rb

+class AddForeignKeyToCiVariables < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    execute <<~SQL
+      DELETE FROM ci_variables
+      WHERE NOT EXISTS (
+        SELECT true
+        FROM projects
+        WHERE projects.id = ci_variables.project_id
+      )
+      OR ci_variables.project_id IS NULL
+    SQL
+
+    add_concurrent_foreign_key(:ci_variables, :projects, column: :project_id)
+  end
+
+  def down
+    remove_foreign_key(:ci_variables, column: :project_id)
+  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: 20170508170547) do
+ActiveRecord::Schema.define(version: 20170508190732) do
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -347,12 +347,12 @@ ActiveRecord::Schema.define(version: 20170508170547) do
   add_index "ci_triggers", ["project_id"], name: "index_ci_triggers_on_project_id", using: :btree
   create_table "ci_variables", force: :cascade do |t|
-    t.string "key"
+    t.string "key", null: false
     t.text "value"
     t.text "encrypted_value"
     t.string "encrypted_value_salt"
     t.string "encrypted_value_iv"
-    t.integer "project_id"
+    t.integer "project_id", null: false
   end
   add_index "ci_variables", ["project_id"], name: "index_ci_variables_on_project_id", using: :btree
@@ -1417,6 +1417,7 @@ ActiveRecord::Schema.define(version: 20170508170547) do
   add_foreign_key "ci_pipelines", "ci_pipelines", column: "auto_canceled_by_id", name: "fk_262d4c2d19", on_delete: :nullify
   add_foreign_key "ci_trigger_requests", "ci_triggers", column: "trigger_id", name: "fk_b8ec8b7245", on_delete: :cascade
   add_foreign_key "ci_triggers", "users", column: "owner_id", name: "fk_e8e10d1964", on_delete: :cascade
+  add_foreign_key "ci_variables", "projects", name: "fk_ada5eb64b3", on_delete: :cascade
   add_foreign_key "container_repositories", "projects"
   add_foreign_key "issue_assignees", "issues", on_delete: :cascade
   add_foreign_key "issue_assignees", "users", on_delete: :cascade