Finding nil has_one associations in where query

Question

This may be a simple question, but I seem to be pulling my hair out to find an elegant solution here. I have two ActiveRecord model classes, with a has_one and belongs_to as

Answer 1

A more concise version of @dimuch solution is to use the left_outer_joins method introduced in Rails 5:

Item.left_outer_joins(:purchase).where(purchases: {id: nil})

Note that in the left_outer_joins call :purchase is singular (it is the name of the method created by the has_one declaration), and in the where clause :purchases is plural (here it is the name of the table that the id field belongs to.)