Use the model's adapter for the attribute type lookup

activerecord/lib/active_record/attributes.rb

@@ -247,7 +247,8 @@ module ActiveRecord
         super
         attributes_to_define_after_schema_loads.each do |name, (type, options)|
           if type.is_a?(Symbol)
-            type = ActiveRecord::Type.lookup(type, **options.except(:default))
+            adapter_name = ActiveRecord::Type.adapter_name_from(self)
+            type = ActiveRecord::Type.lookup(type, **options.except(:default), adapter: adapter_name)
           end
 
           define_attribute(name, type, **options.slice(:default))

activerecord/lib/active_record/type.rb

@@ -46,9 +46,14 @@ module ActiveRecord
         @default_value ||= Value.new
       end
 
+      def adapter_name_from(model) # :nodoc:
+        # TODO: this shouldn't depend on a connection to the database
+        model.connection.adapter_name.downcase.to_sym
+      end
+
       private
         def current_adapter_name
-          ActiveRecord::Base.connection.adapter_name.downcase.to_sym
+          adapter_name_from(ActiveRecord::Base)
         end
     end