with_recursive

Add a recursive Common Table Expression (CTE) that you can then reference within another SELECT statement.

Post.with_recursive(post_and_replies: [Post.where(id: 42), Post.joins('JOIN post_and_replies ON posts.in_reply_to_id = post_and_replies.id')])
# => ActiveRecord::Relation
# WITH RECURSIVE post_and_replies AS (
#   (SELECT * FROM posts WHERE id = 42)
#   UNION ALL
#   (SELECT * FROM posts JOIN posts_and_replies ON posts.in_reply_to_id = posts_and_replies.id)
# )
# SELECT * FROM posts

See `#with` for more information.