Backbone.Validation

var Model = Backbone.Model.extend({
  validation: {
    dependsOn: function(value, attributeName, computedState) {

      // computedState is the model's state after the current operation completes.
      // You need to get the value of the other attribute from this, and not from the
      // model it self because if both attributes are being set at the same time,
      // model's value is not yet updated.
      if(computedState.someAttribute === 'someValue') {

        // If some condition is true, execute the min validator, else don't do anything.
        return Backbone.Validation.validators.min(value, attributeName, 5 /*the min value*/, this);
      }
});