Has Many Relationship: collection_ids= Method Useless?

rubyonrails-talk
1

When modifying a model that has a has_one or has_many relationship people usually create form that will submit the id of the selected associated object. Rails takes this into consideration with validations: validate_presence_of :country will validate if country or country_id has been set.

In Rails, providing the associated ID works fine with has_one but if you try to do this with a has_many:

form_for(@country) do |form|   form.select :state_ids, @states   form.select :state_ids, @states   #etc...

The selected state_ids are written to the DB as soon as they're assigned. What's the reasoning behind this? How is one supposed to assign multiple collection ids via a form?

accepts_nested_attributes_for doesn't work out too well either because you're just setting IDs -a field that is explicitly ignored by accepts_nested except when testing whether or not the given params are new.

2

You have to send them as an array, and assign them as an array too.

For example, View form.select("state_ids", @states) form.select("state_ids", @states)

Controller @whatever.state_ids = params[:state_ids]

You'll have to figure out the exact syntax, but that's the idea.

Hope it helps.

3

That's the problem. As soon as you make that assignment the IDs are written to the db. You'd have to say: Whatever.tranaction do   @what = Whatever.new(params[:whatever])   #other stuff   if @what.valid? .... end

Which can result in an unnecessarily large transaction scope. Though this is somewhat expected since << and co. also write to the DB on assignment.