How I validate if a string exist in mongoose

I have this squema. I need a custom validation like this, but doesn't work, the MonographSchema.path('v1').validate() is never executed.

  var mongoose = require('mongoose'),
    Schema = mongoose.Schema;

  var MonographSchema = new Schema({
        v1: { 
              type: String, 
              trim: true
            },
        v3: {                   
               _: String,
               a: String,
               b: String,
               c: [String],
               t: Number
            }              
  });

 MonographSchema.path('v1').validate(function (value) {
     if(this.v3.a === 'MC'){
        if(!value){
          return false;
        }
     }
     return true;
 }, 'The field "v1" is obligatory');


 mongoose.model('Monograph', MonographSchema);



 var monograph = new Monograph({            
        v3: {'_': 'BR1.1', 'a': 'MC', 'b': 'T17a', 'c': ['v.1', 'e.2'], 't': '1001'}
 });

 monograph.save(); //The value is inserted without problems, the validation isn't trigger.

Is there any way to do this validation to the String in the squema correctly?