AR: dynamic accessors don't always work?

This is curious. Consider 'record' below, derived from an SQL query.
Notice that record.station_id returns the station id properly, but
record.start_time returns nil. What's the difference? (FWIW, I *can*
access the start_time via the construct record[:start_time]).

This took me a while to track down, so I need to know: what's the
difference between the two fields? And - more importantly - what part of
the documentation explains what's going on?

record

=> #<WeatherObservation precipitation_m: 0.0>

record.station_id

=> "KCAMILLV8" # got the station id

record.start_time

=> nil # where's the start time?

record[:start_time]

=> "2007-01-01 00:00:00" # ah - there it is! but wtf?

record.attributes

=> {"end_time"=>"2007-01-02 00:00:00", "precipitation_m"=>0.0,
"start_time"=>"2007-01-01 00:00:00", "station_id"=>"KCAMILLV8",
"temperature_c"=>"8.88889"}

I'm not sure this is germane to the problem, but here's the SQL query
that produced the record. NOTE: neither "station_id" nor "start_time"
are 'native' slots in the WeatherObservation class:

    SELECT st.datetime AS start_time,
           et.datetime AS end_time,
           ws.station_id AS station_id,
           wo.temperature_avg_c AS temperature_c,
           wo.precipitation_m AS precipitation_m
      FROM weather_observations AS wo
INNER JOIN weather_stations AS ws ON wo.weather_station_id = ws.id
INNER JOIN time_dimensions AS st ON wo.start_time_id = st.id
INNER JOIN time_dimensions AS et ON wo.end_time_id = et.id
     WHERE ws.id IN (#{station_ids}) AND
           st.datetime BETWEEN \'#{start_date}\' and \'#{end_date}\' AND
           et.datetime BETWEEN \'#{start_date}\' and \'#{end_date}\'

... all wrapped up in WeatherObservation.find_by_sql(), of course.

what columns does weather_observations have ? From glancing at the
relevant bit of the internals i think an odd thing might happen if
weather_observations itself had a start_time column of its own that
was of a different type to the data grabbed here

Fred

Frederick Cheung wrote:

what columns does weather_observations have ? From glancing at the
relevant bit of the internals i think an odd thing might happen if
weather_observations itself had a start_time column of its own that
was of a different type to the data grabbed here

Fred

I had the same thought, but while WeatherObservation has a
start_time_id, but it doesn't have a start_time slot. Is AR being
clever about foreign key references? Here's the schema:

  create_table "weather_observations", :id => false, :force => true do

t>

    t.integer "start_time_id"
    t.integer "end_time_id"
    t.integer "weather_station_id"
    t.float "temperature_max_c"
    t.float "temperature_avg_c"
    t.float "temperature_min_c"
    t.float "dewpoint_max_c"
    t.float "dewpoint_avg_c"
    t.float "dewpoint_min_c"
    t.float "humidity_max"
    t.float "humidity_avg"
    t.float "humidity_min"
    t.float "pressure_max_kpa"
    t.float "pressure_avg_kpa"
    t.float "pressure_min_kpa"
    t.float "windspeed_max_mps"
    t.float "windspeed_avg_mps"
    t.float "gustspeed_max_mps"
    t.float "precipitation_m"
  end

is there a belongs_to :start_time ?
If so then it's the accessor for that association that's being called,
not the attribute reader (and it's nil because the object at hand has
no start_time_id

Fred

Frederick Cheung wrote:

is there a belongs_to :start_time ?
If so then it's the accessor for that association that's being called,
not the attribute reader (and it's nil because the object at hand has
no start_time_id

Fred

Give this man a cigar! That's exactly what's going on.

class WeatherObservation < ActiveRecord::Base
  belongs_to :start_time, :class_name => 'TimeDimension', :foreign_key
=> 'start_time_id'
  belongs_to :end_time, :class_name => 'TimeDimension', :foreign_key =>
'end_time_id'
  belongs_to :weather_station
...
end

The ActiveRecord::Associations::ClassMethods doc has a nice section on
"auto generated methods" -- I didn't notice that it's the argument to
:belongs_to that defines the method name and not :foreign_key, i.e. it
generates:

   WeatherObservation#start_time
rather than:
   WeatherObservation#start_time_id

Good catch -- thanks.