Add `Column#bigint?` method

activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb

@@ -86,8 +86,8 @@ def schema_creation
       def column_spec_for_primary_key(column)
         spec = {}
         if column.auto_increment?
-          return unless column.limit == 8
-          spec[:id] = ':bigint'
+          spec[:id] = ':bigint' if column.bigint?
+          return if spec.empty?
         else
           spec[:id] = column.type.inspect
           spec.merge!(prepare_column_options(column).delete_if { |key, _| [:name, :type, :null].include?(key) })

activerecord/lib/active_record/connection_adapters/column.rb

@@ -34,6 +34,10 @@ def has_default?
         !default.nil?
       end
 
+      def bigint?
+        /bigint/ === sql_type
+      end
+
       # Returns the human name of the column name.
       #
       # ===== Examples

activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb

@@ -128,7 +128,7 @@ def schema_creation # :nodoc:
       def column_spec_for_primary_key(column)
         spec = {}
         if column.serial?
-          return unless column.sql_type == 'bigint'
+          return unless column.bigint?
           spec[:id] = ':bigserial'
         elsif column.type == :uuid
           spec[:id] = ':uuid'