result = A.joins("INNER JOIN B B1 ON A.id = B1.A_id")
          .joins("INNER JOIN B B2 on A.id = B2.A_id")
          .where("B1.key" => "B1", "B2.key" => "B2")

result.map {|res|
  A = res.A
  B1 = res.B1
  B2 = res.B2
}