backbone.js model validation isValid() returns true for invalid attribute - validation

Question

I was trying to check the validity of individual attributes using isValid method. It is returning true for an invalid attribute. My code is as follows:
person = Backbone.Model.extend({
defaults:{
name:"default name",
age:0
},
initialize:function(){
this.on("invalid",function(model,errors){
console.log(JSON.stringify(errors));
});
},
validate:function(attrs){
errors=[];
if(attrs.age<0){
errors.push({attribName:"age",errorMsg:"age should be grater than 0"});
}
return errors.length>0?errors:false;
}
});
var person1 = new person();
person1.set({
age:-5
});
console.log("checking validity of model:"+person1.isValid());
console.log("checking for validity of age attribute:"+person1.isValid('age'));
isValid() works fine if used to check the validity of the model as a whole and returns false. But when I try to check the age attribute i.e isValid('age') it returns true when it should return false.
isValid() is an underscore.js function, right? Doesn't it support passing an attribute to check for its validity? What am I missing here?

Answer 1

Short version
Model.isValid doesn't accept an attribute name as argument and has to be used on the whole model. If you don't, you're on undocumented territory and you will get weird behaviors.
To check individual attributes, you will have to set up your own mechanism.
Long version, why you get a different value
Model.isValid does in fact accept an (undocumented) options hash as its first argument and it internally forwards this hash to Model._validate via
this._validate({}, _.extend(options || {}, { validate: true }))
trying to set a validate attribute to true. But at this point, options is a string and won't be modified by _.extend. _validate looks like
_validate: function(attrs, options) {
if (!options.validate || !this.validate) return true;
// ...
}
checking if it indeed has to validate the model, options.validate is undefined and your isValid call gets back a true value.

Answer 2

isValid is a backbone API : http://backbonejs.org/#Model-isValid
The reason it is returning true is, the parameter accepted by isValid is an options paramter. It has to be an object.
One of the scenario you use options is :
validate: function(attrs, options) {
if(options.someSpecialCheck) {
// Perform some special checks here
} else {
// Perform some regular checks here
}
}
myModel.isValid({someSpecialCheck: true});