1. A computer system for performing an action on a target model, wherein the target model is associated with a notify model, the target model comprising target objects and the notify model comprising notify objects, the computer system comprising: a model map for mapping the notify objects of the notify model to associated target objects in the target model; and an action operator for performing the action on one or more target objects in the target model in response to a modification of a selected notify object; wherein, the action operator performs the action on one or more identified target objects associated with the modified selected notify object, the one or more identified target objects being determined with reference to the model map.