Fix Sitemap for tags (#14989)

Set a NOT NULL constraint on tags timestamps

This commit also monkeypatches in a method for StrongMigrations to
disable a check temporarily. This migration is safe enough for DEV (it's
not ideal, but it'll only lock the table for a matter of milliseconds),
so we'll be fine disabling it but we don't want to disable the check
globally for all migrations in case there's a migration where this is
not safe.

config/initializers/strong_migrations.rb

@@ -9,3 +9,13 @@ StrongMigrations.target_postgresql_version = 11
 
 # https://github.com/ankane/strong_migrations#down-migrations--rollbacks
 StrongMigrations.check_down = true
+
+module StrongMigrations
+  def self.temporarily_disable_check(check)
+    disable_check check
+
+    yield
+  ensure
+    enable_check check
+  end
+end

db/migrate/20211008170433_add_not_null_constraint_to_tag_timestamps.rb

@@ -0,0 +1,32 @@
+class AddNotNullConstraintToTagTimestamps < ActiveRecord::Migration[6.1]
+  # Timestamps were added to acts_as_taggable_on at this time
+  # See: https://github.com/mbleigh/acts-as-taggable-on/commit/ce14b9ead3a283577a976b6b0d9bfbec278e21bb
+  DEFAULT_TIMESTAMP = "2019-02-06 16:43:19"
+
+  def up
+    # This migration will lock the table, so StrongMigrations is right to call
+    # it out, but this particular table isn't so large that a table lock will
+    # cause downtime. The EXPLAIN ANALYZE on the query that will be used to
+    # check this on DEV shows "Execution Time: 34.136 ms". We can handle a 34ms
+    # table lock.
+    StrongMigrations.temporarily_disable_check :change_column_null_postgresql do
+      Tag
+        .where(created_at: nil)
+        .update_all(created_at: DEFAULT_TIMESTAMP)
+
+      Tag
+        .where(updated_at: nil)
+        .update_all(updated_at: DEFAULT_TIMESTAMP)
+
+      change_column_null :tags, :created_at, false
+      change_column_null :tags, :updated_at, false
+    end
+  end
+
+  def down
+    StrongMigrations.temporarily_disable_check :change_column_null_postgresql do
+      change_column_null :tags, :created_at, true
+      change_column_null :tags, :updated_at, true
+    end
+  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: 2021_10_07_172232) do
+ActiveRecord::Schema.define(version: 2021_10_08_170433) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "citext"
@@ -1189,7 +1189,7 @@ ActiveRecord::Schema.define(version: 2021_10_07_172232) do
     t.bigint "badge_id"
     t.string "bg_color_hex"
     t.string "category", default: "uncategorized", null: false
-    t.datetime "created_at"
+    t.datetime "created_at", null: false
     t.integer "hotness_score", default: 0
     t.string "keywords_for_search"
     t.bigint "mod_chat_channel_id"
@@ -1206,7 +1206,7 @@ ActiveRecord::Schema.define(version: 2021_10_07_172232) do
     t.boolean "supported", default: false
     t.integer "taggings_count", default: 0
     t.string "text_color_hex"
-    t.datetime "updated_at"
+    t.datetime "updated_at", null: false
     t.text "wiki_body_html"
     t.text "wiki_body_markdown"
     t.index ["name"], name: "index_tags_on_name", unique: true