Ruby on Rails : Dynamic Drop down list

First we will look at the html for a basic selection box:

<select name="payment">
<option value="1">VISA</option>
<option value="2">MasterCard</option>
<option value="3">Switch</option>
</select>

It can get quite confusing when it comes to using the different select helpers, select, select_tag and collection_select and in my opinion there is a lack of adequate documentation.

select
select(object, method, choices, options = {}, html_options = {})
Defined in ActionView::Helpers::FormOptionsHelper

º object - name of an instance variable. This is typically a model object (singular name of the table whose data you're displaying, or in other words, the table record).
º method - attribute of that instance variable. This is typically a field/column of the table whose data you're displaying (really an ActiveRecord method).
º choices - can be any enumerable object e.g arrays and hashes and results of database queries, and contains the option tags for the select box.
º options - takes various "options" some of which are listed below in the examples.
º html_options - allows css to be used for styling the select box.

In this example a hash is being used to populate the option tags.

<%= select( "payment", "id", { "Visa" => "1", "Mastercard" => "2"}) %>

Select can be used in conjunction with a model object as seen in this example, an instance variable is passed into choices, but is being converted into an array of arrays.

<%= select ("selected_payment", "id", @payments.map {|u| [u.name,u.id]}) %>

The controller may look something like this:

@payments = Payment.find(:all)
@selected_payment = @payments[2]

This will cause the third option_tag to be selected by default in the select box, but collection_select is preferable in this scenario.
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select_tag
select_tag(name, option_tags = nil, options = {})
Defined in ActionView::Helpers::FormTagHelper

º name - name of the select statement in the generated html, and is used by the controller to access the selected items.
º option_tags - a string containing the option tags for the select box.
º options - takes various "options" some of which are listed below in the examples.

Simple use of the select_tag helper:

<%= select_tag "payment", "<option>VISA</option>" %>

In this example, we are using options_for_select which accepts any container and returns a string of option tags, but we have defined an array. Note how the optional second parameter specifies the selected item in the list.

<%= select_tag "payment", options_for_select([ "VISA", "MasterCard", "Switch" ], "MasterCard") %>

Another option is this

<%= select_tag "payment", options_for_select(%w{ VISA Mastercard Switch }) %>

where %w is the shortcut notation for an array of strings that are space separated.

You can also do multi-select boxes:

<%= select_tag 'payment[]', options_for_select(@payments), :multiple => true, :size => 3 %>

Note that the "name" in this case is an array, and @payments is a hash defined in the controller:

@payments = {'Visa' => 1, 'Mastercard' => 2, 'Switch' => 3}

We can access the selected payment methods in the controller, using the array:

params[:payment]

select_tag does not support the :prompt option, which may not seem like an issue as we have already shown above how you can select a default option tag. But what if you want your form to GET, not POST, your only option is then to use select_tag, even if your data source is a database.

<%= select_tag(:payment, "<option>-Select a payment method</option>" + options_from_collection_for_select(@payments, :name, :name)) %>

As select_tag takes a string, we can concatenate an additional string onto the beginning of the parameter, this allows us to add our default option_tag. We use options_from_collection_for_select which takes a value_method, and text_method (see collection_select for their definition) and an optional selected_value, and a collection, which can be defined in your controller like this:

@payments = Payment.find(:all, :order=>"name")

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collection_select
collection_select(object, method, collection, value_method, text_method, options = {}, html_options = {})
Defined in ActionView::Helpers::FormOptionsHelper

º object - singular name of the table whose data you're displaying (the table record).
º method - field/column of the the relevant data (really an ActiveRecord method).
º collection - takes the option tags for the select box, this can be a hash or array.
º value_method - field/column to use for the value of the option tags in your html.
º text_method - field/column to use for the visible text of the option tags in your html.
º options - takes various "options" some of which are listed below in the examples.
º html_options - allows css to be used for styling the select box.

In this example the payment methods are being added to the select box via a model object, notice how we use the option, :prompt to add an additional option tag to the select box, which will be selected by default. Note that if @object.method matches one of the option tags, this will be selected by default, and :prompt wont appear in the list.

<%= collection_select(:payment, :id, @payments, :id, :name, options ={:prompt => "-Select a payment"}, :class =>"payment") %>

The selected item can be accessed in the controller, by using:

selected_payment = params[payment][id]

A final note: the parameters used in all the helpers may be strings or symbols (symbols may be a better choice in some situations, but that's another article).
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Taken From : http://shiningthrough.co.uk/blog/show/6