code: function (item, opts) {
  		var errors;
  		var allowSet = true;
  
  		// run validation
  		errors = validate.once(item.value, assign({}, opts), item.key);
  
  		// Process errors if we got them
  		if (errors && errors.length > 0) {
  			// Create errors property if doesn't exist
  			if (!this.attr("errors")) {
  				this.attr("errors", new ErrorsObj({}));
  			}
  
  			// Apply error response to observable
  			this.attr("errors").attr(item.key, errors);
  
  			// Don't set value if `mustValidate` is true
  			if (opts.mustValidate === true) {
  				allowSet = false;
  			}
  		} else {
  			// clear errors for this property if they exist
  			if (this.attr("errors") && this.attr("errors").attr(item.key)) {
  				this.attr("errors").removeAttr(item.key);
  			}
  		}
  
  		return allowSet;
  	}
docs: @function _validateOne Validate One
  @hide
  @description Main method used by `Map.define` setter when a property changes.
  Runs validation on a property. Actual behavior of "validate one" is defined
  by the registered shim (`once`).
  
  It also handles setting the errors property on the map instance and then
  manages the errors for the current property within the errors object.
  
  @param {object} item A key/value object
  @param {object} opts Object that contains validation config.
  @return {boolean} True if method found that the property can be saved; if
  validation fails and the property must validate (`mustValidate` property),
  this will be `false`.