Rails 4查询记录，其中嵌套的has_many为空

Question

Working in Rails 4.

CompensationRecipient --has_many--> Compensations --has_many--> Payment

So:

class CompensationRecipient
  has_many :compensations
end

class Compensation
  has_many :payments
  belongs_to :compensation_recipient
end

class Payment
  belongs_to :compensation
end

I want to get all the compensation recipients who's compensations have no payments on them. It seems like it should not be very difficault, but I've not found the solution.

something like this:

CompensationRecipient.includes(compensations: :payments).where(compensations: { payments: { id: nil }} )

That's not working however: PG::UndefinedColumn: ERROR: column compensations.compensation_id does not exist

It is true, that column does not exist. I don't understand why this query expects it though.

Answer 1

You need left outer join in this case. There is no special method for this in rails 4 (in rails 5 we have left_joins ), but you could pass chunk of sql in joins method.

CompensationRecipient
  .joins("LEFT JOIN compensations ON compensations.compensation_recipient_id = compensation_recipient.id")
  .joins("LEFT JOIN payments ON payments.compensation_id = compensations.id")
  .where(payments: { id: nil })

UPD: The first join should be inner though, because you don't want recipients without compensations at all, right?

CompensationRecipient
  .joins(:compensations)
  .joins("LEFT JOIN payments ON payments.compensation_id = compensations.id")
  .where(payments: { id: nil })