Symfony lucene separate search fields
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I have table whit 4 field witch i want to search.
I indexing field like this:

    $doc->addField(Zend_Search_Lucene_Field::Keyword('pk', $this->getId()));
        $doc->addField(Zend_Search_Lucene_Field::UnStored('username', $this->getUsername(), 'utf-8'));
        $doc->addField(Zend_Search_Lucene_Field::UnStored('firstName', $this->getFirstName(), 'utf-8'));
        $doc->addField(Zend_Search_Lucene_Field::UnStored('lastName', $this->getLastName(), 'utf-8'));
        $doc->addField(Zend_Search_Lucene_Field::UnStored('city', $this->getCity(), 'utf-8'));

I want to separate search on this fields whith form Options.

    <select name="contact[subject]" id="contact_subject">
      <option value="0">Username</option>
      <option value="1">First Name</option>
      <option value="2">Last Name</option>
      <option value="3">City</option>
    </select>
How to tell Lucene to search result  only in field who i want ? 
I read many questions and explenations , but i dont get it :( 


