How to query records by matching a parameter within the same record?

Question

In my Rails application, I have a Project model with an associated Task model (Project has many tasks). Each Task has a field called parent_task_id that links it to a task from a previous version. When a new Task is created in a new version, it references itself by setting parent_task_id equal to its own task_id.

I’m trying to write a query to find all tasks in a new Project version where parent_task_id is equal to task_id—this would identify records that were newly added in the latest version.

However, my current query is returning an empty array, even though I know there are matching records. Here’s what I have so far: new_tasks = new_project.tasks.where('tasks.task_id = tasks.parent_task_id')

new_tasks = new_project.tasks.where('tasks.task_id = tasks.parent_task_id')

expected output

{
  task_id: 64741,
  parent_task_id: 64741,
  # other attributes...
}

How can I correctly structure this ActiveRecord query to find tasks where parent_task_id equals task_id? Is there a special consideration in ActiveRecord for self-referencing conditions like this?

Any help or tips would be much appreciated. Thank you!

Answer 1

Use Arel, which is built in to Rails, to compose your query instead of the query string:

table = Arel::Table.new(:tasks)
new_tasks = new_project.tasks.where(t[:task_id].eq(t[:parent_task_id]))