code: public static Field keyField(Class<?> clazz) {
  		for (Field field : clazz.getDeclaredFields()) {
  			if (field.getAnnotation(Key.class) != null) {
  				field.setAccessible(true);
  				fields.put(clazz, field);
  				return field;
  			}
  		}
  		
  		throw new IllegalArgumentException("Entity [" + clazz
  				+ "] must have one @org.eiichiro.acidhouse.Key field");
  	}
docs: Returns key field of the specified entity class.
  
  @param clazz An entity class.
  @return The key field of the specified entity class.