ids writer fields for HABTM relationship.

Hello forum readers,

Let's say i have two models: Person and Role (i replicated the problem with Post & Tag as well, as i thought the problem was linked to some application-specific rights problem, but it wasn't).

In Person, attr_accessible role_ids is declared, so it accepts an array of Role ids as an argument when creating / updating.

If i fire up the rails console and just do: [code] Person.first.role_ids = [1, 2, 3] [/code] Everything works so far.

Now from the web browser, when trying to update a specific person, i get: [code] Started PUT "/people/1?_dc=1351904653950" for at 2012-11-02 18:04:13 -0700 Processing by PeopleController#update as JSON   Parameters: {"email"=>"", "last_name"=>"DOE", "first_name"=>"John", "company_id"=>1, "created_at"=>"2012-11-02T22:27:00Z", "updated_at"=>"2012-11-02T22:27:00Z", "role_ids"=>[2, 1], "id"=>"1", "_dc"=>"1351904653950", "person"=>{"email"=>"", "first_name"=>"John", "last_name"=>"DOE", "role_ids"=>[2, 1]}} WARNING: Can't verify CSRF token authenticity   Person Load (0.2ms) SELECT "people".* FROM "people" WHERE "people"."email" = '' LIMIT 1   Role Load (0.1ms) SELECT "roles".* FROM "roles" INNER JOIN "people_roles" ON "roles"."id" = "people_roles"."role_id" WHERE "people_roles"."person_id" = 1   ModelRight Load (0.1ms) SELECT "model_rights".* FROM "model_rights" WHERE "model_rights"."role_id" IN (2, 1, 3)   Company Load (4.5ms) SELECT "companies".* FROM "companies" WHERE "companies"."id" = ? LIMIT 1 [["id", 1]]   Person Load (0.2ms) SELECT "people".* FROM "people" WHERE "people"."company_id" = 1 AND "people"."id" = ? LIMIT 1 [["id", "1"]]    (0.1ms) begin transaction    (0.0ms) rollback transaction Completed 500 Internal Server Error in 2790ms NoMethodError (undefined method `type_cast' for nil:NilClass):   app/controllers/people_controller.rb:21:in `update' [/code]

I scratched my head for quite a long time before finding out that after restarting the rails server, the error would disappear and the Person would actually be updated...

So with the help of pry and pry-debugger, i have pinpointed the exact source of the "NoMethodError" error (in collection_association.rb, at line 69): [code]      # Implements the ids writer method, e.g. foo.item_ids= for Foo.has_many :items      def ids_writer(ids)        pk_column = reflection.primary_key_column        ids = Array.wrap(ids).reject { |id| id.blank? }! { |i| pk_column.type_cast(i) }        replace(klass.find(ids).index_by { |r| }.values_at(*ids))      end [/code] This method gets called by the People controller in the update action (@person.update_attributes params[:person]), which in turn deeper in Rail's guts arrive to the pinpointed method when accessing an *_ids writer...

What is very weird, is that pk_column is nil (Hence the NoMethodError error later), but when restarting the server and trying to output pk_column, then it is a valid association column... Random behavior, basically.

Is there anyone who would help me to understand what i'm doing wrong? Is a *_ids writer field any different in an HABTM assotiation compared to a regular HM association?

Thank you for your help!

Best, Pierre.