使用Rails中的联接表的模型进行左外部联接

Question

I need to do a left outer join in rails, but I need the model objects to be for the joined table.

What I want is a list of the days, with the metrics for each day. I need to have all days regardless of whether or not there were metrics, but I don't want to make a bunch of round trips to the database.

This works, but causes problems because it thinks I have PeriodDay objects when I really want Metric objects:

PeriodDay.select("metrics.*").join('LEFT OUTER JOIN metrics ON period_days.date = metrics.date').where('period_id = ?', current_period)

I can use find_by_sql on the Metric object, but the query building is more complicated (and conditional) than this simplified example, so I would rather figure out the "rails way" for this problem.

Answer 1

My current workaround is to loop through the records and create Metric objects from the attributes of the PeriodDay object. It doesn't feel efficient, but it is better than making multiple database calls.

metrics = []
recs = PeriodDay.select("metrics.*").join('LEFT OUTER JOIN metrics ON period_days.date = metrics.date').where('period_id = ?', current_period)
for rec in recs
  metrics << Metric.new(rec.attributes)
end

Answer 2

Assuming that Period has many PeriodDay has many Metric, and that period_id is an attribute of your PeriodDay model, your workaround should be identical to something like this:

Metric.includes(:period_day).where(:period_day => {:period_id => @current_period})

This doesn't get you a list of days with their respective Metric objects as you mentioned in the original question, but it gets you a list of all Metric objects for a particular period. (unless I'm missing something...)

If you want a list of PeriodDay objects with their included Metric objects, you can use includes instead of joins .

PeriodDay.includes(:metrics).where(:period_id => @current_period)

This will execute two queries (one to get period days and the other to get metrics) but it is a lot more readable.