Multiple select issue with a HABTM relationship using Rails 4

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.

Do you have any ideas about what could be happening here?

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>

``

Are you sure about that? Check the log output from submitting the
form.