Qualitative approaches, like Piecewise-Affine Differential Equations (PADEs) or those inspired from the R. Thomas formalism, represent one of the major recent improvements in biological modeling. We show herein that these approaches might be naturally represented within a unified theoretical framework using constraints. This result allows us to reason about biological models which is helpful for (i) passing from one qualitative formalism to another one and as well for (ii) building a constraints-based protocol that opens perspectives on modeling large genetic regulatory systems