Pass selected value to Rails controller - ruby-on-rails

Question

I've got a method named 'statement' in a balances controller (balances_controller.rb) that returns information from a specific date (#latestDate) to a view (statement.html.rb)
#latestDate = the most recent date in the 'date' column in the 'balances' table
In the view, I've also got a dropdown list of all the dates and when a date is selected, I'd like to be able to update the information so that in effect, the statement method is called again but the #latestDate is set to the selected value from the dropdown list.
How can I do this?
UPDATE:
I've modified my code with brito's suggestions. When I press the Search button, the form passes the following params:
?utf8=✓&date_id=30%2F12%2F2015&commit=Search
However, if I select a date and click Search, the #latestDate gets set and the h1 tag gets displayed correctly, but the rest of the data doesn't get returned.
CODE:
balances_controller.rb
....
def statement
#dates = Balance.select("distinct(date)").order('date desc')
#Updated
if (params[:date_id].blank?)
#latestDate = Balance.order('date desc').first.date
else
#latestDate = params[:date_id]
end
#summaryBalances = Balance.joins(:account).order('accounts.credit').where('date = :abc', {abc: #latestDate})
end
....
balances/statement.html.rb
....
<h1>Statement at <%= #latestDate %></h1>
#UPDATED
<%= form_tag("/statement", method: "get") do %>
<%= select_tag "date_id", options_for_select( #dates.collect {|d| [ d.date, d.date ] }) %>
<%= submit_tag("Search") %>
<% end %>
<div class="table-responsive">
<table class="table">
<thead>
<tr>
<th>Account</th>
<th>Balance</th>
<th colspan="1"></th>
</tr>
</thead>
<tbody>
<% #summaryBalances.each do |balance| %>
<tr>
<td><%= balance.account.name %></td>
<td class='account-balance'><%= number_to_currency(balance.balance, unit: "£") %></td>
<td><%= link_to 'Edit', edit_balance_path(balance) %></td>
</tr>
<% end %>
</tbody>
</table>
</div>
....

Answer 1

A "dropdown" is a kind of input (HTML <select>).
<input>s and <select>s must be within a <form> HTML element.
Using pure HTML
Whatever is in the <form> will be accessible in the params hash. So if you have a dropdown list, it should be within a <form> like this:
<form accept-charset="UTF-8" action="/yourroute" method="get">
<select name="car">
<option value="volvo">Volvo</option>
<option value="saab">Saab</option>
<option value="opel">Opel</option>
<option value="audi">Audi</option>
</select>
</form>
In the example above, the selected value (e.g. "volvo") will be available in the controller within params["car"]. Note that you would still have to define /yourroute.
Using Rails' helper methods
Of course, Rails helps you generate forms and inputs. You can use Rails' form helpers for that. For example, the following generates a search form ("Search" label, text box for input and a button) that sends a GET to the /search route:
<%= form_tag("/search", method: "get") do %>
<%= label_tag(:q, "Search for:") %>
<%= text_field_tag(:q) %>
<%= submit_tag("Search") %>
<% end %>
EDIT
To check and set #latestDate = balance[date_id] if the balance[date_id] was selected:
#latestDate ||= params[:balance[date_id]] unless params[:balance].blank?