Move length value mapping to constant in ids to bigints migration (#29048)

db/migrate/20170918125918_ids_to_bigints.rb

@@ -7,10 +7,7 @@ class IdsToBigints < ActiveRecord::Migration[5.1]
   include Mastodon::MigrationHelpers
   include Mastodon::MigrationWarning
 
-  disable_ddl_transaction!
+  TABLE_COLUMN_MAPPING = [
-
-  def migrate_columns(to_type)
-    included_columns = [
     [:account_domain_blocks, :account_id],
     [:account_domain_blocks, :id],
     [:accounts, :id],
@@ -70,17 +67,13 @@ class IdsToBigints < ActiveRecord::Migration[5.1]
     [:users, :id],
     [:web_settings, :id],
     [:web_settings, :user_id],
-    ]
+  ].freeze
-    included_columns << [:deprecated_preview_cards, :id] if table_exists?(:deprecated_preview_cards)
 
-    migration_duration_warning(<<~EXPLANATION)
+  disable_ddl_transaction!
-      This migration has some sections that can be safely interrupted
-      and restarted later, and will tell you when those are occurring.
 
-      For more information, see https://github.com/mastodon/mastodon/pull/5088
+  def migrate_columns(to_type)
-    EXPLANATION
+    display_warning
 
-    tables = included_columns.map(&:first).uniq
     table_sizes = {}
 
     # Sort tables by their size
@@ -103,6 +96,25 @@ class IdsToBigints < ActiveRecord::Migration[5.1]
     end
   end
 
+  def display_warning
+    migration_duration_warning(<<~EXPLANATION)
+      This migration has some sections that can be safely interrupted
+      and restarted later, and will tell you when those are occurring.
+
+      For more information, see https://github.com/mastodon/mastodon/pull/5088
+    EXPLANATION
+  end
+
+  def tables
+    included_columns.map(&:first).uniq
+  end
+
+  def included_columns
+    TABLE_COLUMN_MAPPING.dup.tap do |included_columns|
+      included_columns << [:deprecated_preview_cards, :id] if table_exists?(:deprecated_preview_cards)
+    end
+  end
+
   def up
     migrate_columns(:bigint)
   end