mechanize：Mechanize::Form#add_field!

以下を確認

lib/mechanize/form.rb

    def add_field!(field_name, value = nil)
      fields << Field.new({'name' => field_name}, value)
    end

    def []=(field_name, value)
      f = field(field_name)
      if f.nil?
        add_field!(field_name, value)
      else
        f.value = value
      end
    end

ということは、以下のような HTML で

<html>
<head>
<title>form test</title>
</head>
<body>

<form method="post" action="cgi/foo00.cgi" name="form1">
</form>

<form method="post" action="cgi/foo01.cgi" name="form2">
</form>

</body>
</html>

>> require 'mechanize'
=> true
>> agent = Mechanize.new
=> #<Mechanize:0xb7495540 ...
>> page = agent.get('http://127.0.0.1:8080/test01_form.html')
=> #<Mechanize::Page
 {url #<URI::HTTP:0xb7491c88 URL:http://127.0.0.1:8080/test01_form.html>}
 {meta}
 {title "form test"}
 {iframes}
 {frames}
 {links}
 {forms
  #<Mechanize::Form
   {name "form1"}
   {method "POST"}
   {action "cgi/foo00.cgi"}
   {fields}
   {radiobuttons}
   {checkboxes}
   {file_uploads}
   {buttons}>
  #<Mechanize::Form
   {name "form2"}
   {method "POST"}
   {action "cgi/foo01.cgi"}
   {fields}
   {radiobuttons}
   {checkboxes}
   {file_uploads}
   {buttons}>}>

>> f = page.form('form1')
  ...
>> f['hoge']
=> nil
>> f['hoge'] = 'aaa'
=> "aaa"
>> f['hoge']
=> "aaa"
>> f
=> #<Mechanize::Form
 {name "form1"}
 {method "POST"}
 {action "cgi/foo00.cgi"}
 {fields
  #<Mechanize::Form::Field:0xb747fe98
   @name="hoge",
   @node={"name"=>"hoge"},
   @value="aaa">}
 {radiobuttons}
 {checkboxes}
 {file_uploads}
 {buttons}>