Wrong name of the column in Rails app with PostgreSQL database

I have association for my School model:

class School < ActiveRecord::Base

  belongs_to :city
  has_many :sclasses
  has_many :users, through: :sclasses

  validates :name, presence: true
  validates :icon_url, length: { maximum: 100 }

end

And this is the part of schools#show view which makes me a problem:

<%= search_form_for @search, url: school_path(@city, @school) do |f| %>
    <div class="row input-field">
        <div class="text-center">
            <%= f.text_field :full_name_cont, placeholder: "Pretraga učenika" %>
        </div>
    </div>
    <div class="row input-field">
        <div class="text-center">
            <%= f.select :generation_eq, options_from_collection_for_select(@school.users.order("generation DESC").select(:generation).uniq, "generation", "generation", @search.generation_eq), {}, { :class => "selectpicker" } %>
        </div>
    </div>
    <div class="row input-field">
        <div class="text-center">
            <%= f.submit "Traži", class: "btn btn-default" %>
        </div>
    </div>
<% end %>

In this part I have the form for searching users which was there when structure of models looked like this:

City has_many School has_many Users

Now it looks like this:

City has_many School has_many Sclasses has_many Users

And I get this error:

PG::UndefinedColumn: ERROR:  column users.sclass_id does not exist
LINE 1: ...generation" FROM "users" INNER JOIN "sclasses" ON "users"."s...
                                                             ^
HINT:  Perhaps you meant to reference the column "users.sclass".
: SELECT DISTINCT "users"."generation" FROM "users" INNER JOIN "sclasses" ON "users"."sclass_id" = "sclasses"."id" WHERE "sclasses"."school_id" = $1  ORDER BY generation DESC

How should I change parameter in the f.select to make this work properly?