Question

We recently upgraded our rails app from version 3.0.3 to 3.1.0. The application runs successfully for the most part as it did before with one major exception. We have a many-to-many relationship between two models, SurveyDatum and SubGroup, joined via a model called SubGroupSurveyDatum. Here is the code for each:

class SurveyDatum < ActiveRecord::Base
  has_many :sub_group_survey_data
  has_many :sub_groups, :through => :sub_group_survey_data
end


class SubGroup < ActiveRecord::Base
  has_many :sub_group_survey_data
  has_many :survey_data, :through => :sub_group_survey_data
end

And as you might expect:

class SubGroupSurveyDatum < ActiveRecord::Base
  belongs_to :survey_datum
  belongs_to :sub_group
end

If I have a SurveyDatum object that I retrieved previously from the database (lets call it ‘sd’), and I invoke the sub_groups method (sd.sub_groups), this is the resulting sql query generated by active record:

SELECT `sub_groups`.* FROM `sub_groups` INNER JOIN `sub_group_survey_data` ON `sub_groups`.`id` = `sub_group_survey_data`.`sub_group_id` WHERE `sub_group_survey_data`.`survey_datum_id` IS NULL

The “IS NULL” part is obviously where the id of my survey data object is supposed to go, however active record fails to use it. The object does indeed have an id, since as mentioned it was persisted and retrieved from the database. This problem only cropped up after we moved to rails 3.1, so I assume there’s something I’ve not done properly in accordance with the new version, but I have no idea. Any ideas? Thank you in advance for your help!

Answer 1

We discovered the issue. I had forgotten that the survey_data table has a composite primary key. When we upped to version 3.2.3, and added in the SurveyDatum model the following:

set_primary_key :id

The query finally built and executed properly.