code: def _get_dynamic_field_for(cls, field_name):
          
          from .fields import DynamicFieldMixin  # here to avoid circular import
  
          if cls not in ModelWithDynamicFieldMixin._dynamic_fields_cache:
              ModelWithDynamicFieldMixin._dynamic_fields_cache[cls] = {}
  
          if field_name not in ModelWithDynamicFieldMixin._dynamic_fields_cache[cls]:
              ModelWithDynamicFieldMixin._dynamic_fields_cache[cls][field_name] = None
              for a_field_name in cls._fields:
                  field = cls.get_field(a_field_name)
                  if isinstance(field, DynamicFieldMixin) and field._accept_name(field_name):
                      ModelWithDynamicFieldMixin._dynamic_fields_cache[cls][field_name] = field
                      break
  
          field = ModelWithDynamicFieldMixin._dynamic_fields_cache[cls][field_name]
  
          if field is None:
              raise ValueError('No DynamicField matching "%s"' % field_name)
  
          return field
docs: Return the dynamic field within this class that match the given name.
          Keep an internal cache to speed up future calls wieh same field name.
          (The cache store the field for each individual class and subclasses, to
          keep the link between a field and its direct model)