Learning environments for object-oriented modelling in UML which offer a rich interaction usually impose, in return, strong restrictions on the range of exercises they can address. We propose to overcome this limit by including a diagnostic module that compares the student diagram with a reference diagram. This approach enables to combine the advantages of an open environment (in which the teacher can add new exercises without constraints on the vocabulary or the size of the diagram) with a sophisticated interaction that offers methodological help, encourages self-correcting and self-monitoring, and provides the learner with specific graphic tools. These principles have been developed and implemented through a learning environment for UML class diagrams. Experiments have been conducted in ecological context and show that this approach is achievable and quite effective.