Metamodelling plays an important role in model-driven engineering as it can be used to define domain-specific modelling languages. During the modelling phase, software designers encode domain knowledge into models which may include both structural and behavioural aspects of a system. In this paper we propose a diagrammatic approach to aid the software designer to complete partial models and thereby reduce modelling effort. We introduce a declarative approach where we define completion rules that are executed by the use of model transformations. We also study the termination of such model transformation systems and provide sufficient conditions for termination by generalizing existing work on termination of model transformation systems.