Discrete combinatorial problems can be solved with Constraint Programming (CP) as long as they are formulated as Constraint Satisfaction Problems (CSP). In this paper we propose an object oriented model to solve combinatorial problems of resource assignment including applications in industry, commerce, and general organizations. Problems of these environments are those having entities that have to be assigned to places. A particular case of these problems is proposed and modeled here. This problem, named the Classroom Problem, is in a school with teachers; each teacher is to be assigned to any of the rooms of the school in different schedules and days. Besides there is a set of constraints limiting such assignment. The advantages with respect to other approaches which deal with a particular case of the problem considered in this work are identified and discussed.