What Does ActiveRecord::MultiparameterAssignmentErrors Mean?

Question

I have a rails form with a datetime_select field. When I try to submit the form, I get the following exception:

ActiveRecord::MultiparameterAssignmentErrors in WidgetsController#update 
1 error(s) on assignment of multiparameter attributes

If it's a validation error, why don't I see an error on the page?

This is in Rails 2.0.2

Answer 1

It turns out that rails uses something called Multi-parameter assignment to transmit dates and times in small parts that are reassembled when you assign params to the model instance.

My problem was that I was using a datetime_select form field for a date model field. It apparently chokes when the multi-parameter magic tries to set the time on a Date object.

The solution was to use a date_select form field rather than a datetime_select.

Answer 2

Super hack, but I needed to solve this problem right away for a client project. It's still a bug with Rails 2.3.5.

Using either date_select or datetime_select, if you add this to your model in the initialize method, you can pre-parse the passed form-serialized attributes to make it work:

def initialize(attributes={})
  date_hack(attributes, "deliver_date")
  super(attributes)
end

def date_hack(attributes, property)
  keys, values = [], []
  attributes.each_key {|k| keys << k if k =~ /#{property}/ }.sort
  keys.each { |k| values << attributes[k]; attributes.delete(k); }
  attributes[property] = values.join("-")
end

I am using this with a nested, polymorphic, model. Here's a question I had showing the models I'm using. So I needed accepts_nested_attributes_for with a datetime.

Here's the input and output using the console:

e = Event.last
=> #<Event id: 1052158304 ...>
e.model_surveys
=> []
e.model_surveys_attributes = [{"survey_id"=>"864743981", "deliver_date(1i)"=>"2010", "deliver_date(2i)"=>"2", "deliver_date(3i)"=>"11"}]
PRE ATTRIBUTES: {"survey_id"=>"864743981", "deliver_date(1i)"=>"2010", "deliver_date(2i)"=>"2", "deliver_date(3i)"=>"11"}
# run date_hack
POST ATTRIBUTES: {"survey_id"=>"864743981", "deliver_date"=>"2010-2-11"}
e.model_surveys
=> [#<ModelSurvey id: 121, ..., deliver_date: "2010-02-11 05:00:00">]
>> e.model_surveys.last.deliver_date.class
=> ActiveSupport::TimeWithZone

Otherwise it was either null, or it would throw the error:

1 error(s) on assignment of multiparameter attributes

Hope that helps, Lance

Answer 3

This is not a bug in Rails, it is the intended behavior of the multi-parameter attribute writer. I'm willing to bet that the original poster's deliver_date field in the database is a varchar as opposed to a date or datetime type. ActiveRecord uses each part of the multi-parameter attribute to send to the new method of the serialized type. The number 1, 2, 3, etc indicates the constructor parameter position and the "i" tells ActiveRecord to call to_i on the parameter before passing it to the constructor. In this case they are all "i's" because DateTime.new(year, month, day) expects three Integers not three Strings.

If the deliver_date column in the database isn't a type that's serialized to a DateTime then ActiveRecord will throw a ActiveRecord::MultiparameterAssignmentErrors exception because String.new(2010,2,11) won't be successful.

Source: https://github.com/rails/rails/blob/v3.0.4/activerecord/lib/active_record/base.rb#L1739

Answer 4

ActiveRecord throws the MultiparameterAssignmentErrors exception when you try to set an invalid date to a models attribute.

Try to pick a Nov 31 date from the date_select or datetime_select dropdown and you will get this error.