Although the code seems to be right, when I try to send the form, the values of the multiple select aren't being sent.
If I just remove the multiple option, everything works as expected considering just one value, but it's important to store more than one tag per transaction.
Model
Transaction.rb
class Transaction < ActiveRecord::Base
has_and_belongs_to_many :tags
Tag.rb
class tag < ActiveRecord::Base
has_and_belongs_to_many :transactions
View
<%= form.collection_select :tag_ids, @tags, :id, :name, {},
{:multiple => true} %>
Result:
<select id="transaction_tag_ids" multiple="multiple" name="transaction[tag_ids][]">
<option value="1">..</option>
</select>
Be sure what you properly allow received params for mass assignment.
You said that the one param works, so I assume what you have somewhere in your controller something like:
params.require(:transaction).permit(:name, :tag_ids)
So you need allow to receive an array:
params.require(:transaction).permit(:name, :tag_ids => [])