Display a checkbox list instead of multiple select

There is another solution worth mentioning that makes it very easy to insert records into the database if you have a has_and_belongs_to_many or has_many through relationship by using the collection_check_boxes form helper. See documentation here.

<%= f.collection_check_boxes :mymodel_ids, MyModel::AS, :id, :name do |m| %>
  <%= m.check_box %> <%= m.label %>
<% end %>

Then, in the controller we would allow the mymodel_ids attribute:

params.require(:mymodel).permit(:name, mymodel_ids:[])

We can also set up a model validation to require that at least one of the checkboxes be checked:

validates :mymodel_ids, presence: true

An added benefit of this method is that if you later edit the mymodel record and uncheck one of the checkboxes, its record will be deleted from the many_to_many association table on save.

You can do it like this:

<% MyModel::AS.each do |a_value| %>
  <%= f.check_box("a[]", a_value) %>  
<% end %>

This will make params come to server as follows

{ :my_model => { :a => [:a_value1, :a_value2] } }

I found a solution, using 'multiple' option that I didn't know.

<% MyModel::AS.each do |a_value| %>
  <%= f.check_box(:a, { :multiple => true }, a_value) %>
<% end %>

Result parameters are a little weird, but it should work.

{"my_model" => { "a" => ["0", "a_value1", "0", "a_value2", "0"] }

Edit from @Viren : passing nil at the end of the function like

  <%= f.check_box(:a, { :multiple => true }, a_value, nil) %>

works perfectly.