Ruby on Rails

I am trying to retrieve only one field instead of all field. Hence guides.rubyonrails.org suggest to use Model.select("field_name, separated_by, comma") here. But the thing is this will produce the SQL Select field_name FROM Model. I want to extend this to include where clause.

Why I want to do this

This is the code in my index page

<tbody>

<% @users.each do |user| %>

<% rolename = Role.where(id: user.roleid).pluck(:role_name) %>

<tr>

<td><%= user.userid %></td>

<td><%= user.fname %></td>

<td><%= user.lname %></td>

<td><%= user.email %></td>

<td><%= user.phno %></td>

<td><%= rolename %></td>

<td><%= link_to 'Show', user ,:class => 'btn btn-default' %></td>

<td><%= link_to 'Edit', edit_user_path(user),:class => 'btn btn-default' %></td>

<td><%= link_to 'Delete', user, method: :delete, data: { confirm: 'Are you sure?' } ,:style => 'color:#FFFFFF', :class => 'btn btn-danger'%></td>

</tr>

<% end %>

</tbody>

The large font code after loop is giving the result what I want but the result is enclosed with big brackets and double quotes like this [" result"]. Because that code will return an array of values.

So I found I have to use Model.select to retrieve single object. Then I tried <% rolename = Role.select("role_name").where(id: user.roleid) %> but its returning some invalid value like 00x30000658487.

Again I tried with this <% rolename = Role.find_by_sql("SELECT role_name FROM ROLES, USERS WHERE roles.id = users.roleid.to_i") %>, its giving error PG::UndefinedTable: ERROR: missing FROM-clause entry for table "roleid"

By comparing all types of queries I thought it is good to include where clause along with Model.select. But couldn't find anywhere.

Please help me

Thank you