Starting a variable name with $ is required in PHP, but pretty weird (IMHO) in Javascript. Also, I believe you refer to it as "$module" twice and "module" once, right? It seems that this code shouldn't work.

Also, I'm not sure if it's normal jQuery plugin syntax, but I might add comments above your addMethod call, explaining what you accomplish. Even with your text description above, it's hard to follow the code, because I'm not familiar with what fieldset, :filled, value, element, or selector refer to. Perhaps most of this is obvious to someone familiar with the Validate plugin, so use judgment about what is the right amount of explanation.

Perhaps you could break out a few vars to self-document the code; like,

var atLeastOneFilled = module.find(...).length > 0;
if (atLeastOneFilled) {
var stillMarkedWithErrors = module.find(...).next(...).not(...);
stillMarkedWithErrors.text("").addClass(...)

(assuming I did understand the meaning of these chunks of your code! :) )

I'm not exactly sure what "module" means, actually -- is there a more specific name you could give to this variable?

Nice code, overall!

That's an excellent solution Nathan. Thanks a lot.

Here's a way making the above code work, in case someone runs into trouble integrating it, like I did:

Code inside the additional-methods.js file:

jQuery.validator.addMethod("require_from_group", function(value, element, options) {
...// Nathan's code without any changes
}, jQuery.format("Please fill out at least {0} of these fields."));


// "filone" is the class we will use for the input elements at this example
jQuery.validator.addClassRules("fillone", {
require_from_group: [1,".fillone"]
});

Code inside the html file:

<input id="field1" class="fillone" type="text" value="" name="field1" />
<input id="field2" class="fillone" type="text" value="" name="field2" />
<input id="field3" class="fillone" type="text" value="" name="field3" />
<input id="field4" class="fillone" type="text" value="" name="field4" />

Don't forget to include additional-methods.js file!

Because the form I'm working on has several cloned regions with grouped inputs like these, I passed an extra argument to the require_from_group constructor, changing exactly one line of your addMethod function:

var commonParent = $(element).parents(options[2]);

and this way a selector, ID or element name can be passed once:

jQuery.validator.addClassRules("reqgrp", {require_from_group: [1, ".reqgrp", 'fieldset']});

and the validator will restrict the validation to elements with that class only inside each fieldset, rather than try to count all the .reqgrp classed elements in the form.

Thanks, Nathan. You saved me a ton of time.

However, i must notice that this rule isn't jQuery.noConflict() ready. So, one must replace all $ with jQuery to work with, say, var $j = jQuery.noConflict()

And i have question: how would i make it behave like built-in rule? For example, if i enter email, the message "Please enter valid email" disappears automatically but if i fill one of group fields error message stays.

Nice solution. However, I had the problem of other required rules not working. Executing .valid() against the form fixed this issue for me.

if(!$(element).data('being_validated')) {
var fields = $(selector, element.form);
fields.data('being_validated', true);
$(element.form).valid();
fields.data('being_validated', false);
}

Thanks sean. That fixed the issue I had with the code ignoring other rules.

I also made a few changes so that 'Please fill out at least 1 field ..' message shows in a separate div instead of after all every field.

put in form validate script

showErrors: function(errorMap, errorList){
$("#form_error").html("Please fill out at least 1 field before submitting.");
this.defaultShowErrors();
},

add this somewhere in the page

<div class="error" id="form_error"></div>

add to the require_from_group method addMethod function

 if(validOrNot){
$("#form_error").hide();
}else{
$("#form_error").show();
}
......
}, jQuery.format("  (!)"));

I was having problems with other rules not being checked in conjunction with this, so I changed:

fields.valid();

To this:

var validator = this;
fields.each(function(){
validator.valid(this);
});

I also made a few (personal) improvements, and this is the version I'm using:

jQuery.validator.addMethod("require_from_group", function(value, element, options){
var numberRequired = options[0],
selector = options[1],
$fields = $(selector, element.form),
validOrNot = $fields.filter(function() {
return $(this).val();
}).length >= numberRequired,
validator = this;
if(!$(element).data('being_validated')) {
$fields.data('being_validated', true).each(function(){
validator.valid(this);
}).data('being_validated', false);
}
return validOrNot;
}, jQuery.format("Please fill out at least {0} of these fields."));

Here's my crack at Rocket Hazmat's answer, trying to solve the issue of other defined fields also needing to be validated, but marking all fields as valid on successful filling of one.

jQuery.validator.addMethod("require_from_group", function(value, element, options){
var numberRequired = options[0],
selector = options[1],
$fields = $(selector, element.form),
validOrNot = $fields.filter(function() {
return $(this).val();
}).length >= numberRequired,
validator = this;
if(!$(element).data('being_validated')) {
$fields.data('being_validated', true).each(function(){
validator.valid(this);
}).data('being_validated', false);
}
if (validOrNot) {
$(selector).each(function() {
$(this).removeClass('error');
$('label.error[for='+$(this).attr('id')+']').remove();
});
}
return validOrNot;
}, jQuery.format("Please fill out at least {0} of these fields."));

The only remaining issue with this now is the edge case where the field is empty, then filled, then empty again... in which case the error will by applied to the single field not the group. But that seems so unlikely to happen with any frequency and it still technically works in that case.

I've submitted a patch that doesn't suffer from the issues the current version does (whereby the "required" option stops working properly on other fields, a discussion of the problems with the current version is on github.

Example at http://jsfiddle.net/f887W/10/

jQuery.validator.addMethod("require_from_group", function (value, element, options) {
var validator = this;
var minRequired = options[0];
var selector = options[1];
var validOrNot = jQuery(selector, element.form).filter(function () {
return validator.elementValue(this);
}).length >= minRequired;


// remove all events in namespace require_from_group
jQuery(selector, element.form).off('.require_from_group');


//add the required events to trigger revalidation if setting is enabled in the validator
if (this.settings.onkeyup) {
jQuery(selector, element.form).on({
'keyup.require_from_group': function (e) {
jQuery(selector, element.form).valid();
}
});
}


if (this.settings.onfocusin) {
jQuery(selector, element.form).on({
'focusin.require_from_group': function (e) {
jQuery(selector, element.form).valid();
}
});
}


if (this.settings.click) {
jQuery(selector, element.form).on({
'click.require_from_group': function (e) {
jQuery(selector, element.form).valid();
}
});
}


if (this.settings.onfocusout) {
jQuery(selector, element.form).on({
'focusout.require_from_group': function (e) {
jQuery(selector, element.form).valid();
}
});
}


return validOrNot;
}, jQuery.format("Please fill at least {0} of these fields."));