Strange problem with query

Hi all,
suppose I have the following model:
class FrotaEmpresa < ActiveRecord::Base
  belongs_to :servico, :foreign_key=>'sev_codigo'
  belongs_to :tipo_veiculo, :foreign_key=>'tve_codigo'
end

in my controller I'm building a query like this:

select = "fro_codigo, f.for_cod, fro_num_ordem, fro_placa,
fro_num_sentados, fro_num_pe, fro_data_vistoria,
fro_chassi, fro_ano_chassi, fro_ano_fabricacao, fro_situacao,
tb_fro_frota_onibus.created_at,
for_num_transportadora, for_cnpj, for_razao_social, for_logradouro,
for_bairro, for_cidade, for_uf, for_cep, sv.sev_codigo,
sv.sev_descricao, mar_descricao,
moc_descricao, tic_descricao, tve_descricao, mcc_descricao,
tcc_descricao"

joins =[" inner join TB_SEV_SERVICO_VEICULO SV on SV.SEV_CODIGO =
TB_FRO_FROTA_ONIBUS.SEV_CODIGO" +
          " inner join TB_TVE_TIPO_VEICULO TV on
TB_FRO_FROTA_ONIBUS.TVE_CODIGO = TV.TVE_CODIGO" +]

conditions = "fro_situacao<>0"

FrotaEmpresa.find(:all,
        :select => select,
        :joins => joins,
        :conditions => conditions,
        :order=>"tb_fro_frota_onibus.for_cod, fro_num_ordem")

Strangely,my find method executes a "select * from" in the
"FrotaEmpresa" table for
each "FrotaEmpresa" returned in the query which contains the joins. How
can I solve this?
Regards,
Rafael Roque