Add "Finding missing relationship records" rule

[ydah]

If you're using Rails 6.1 or higher, prefer where.missing over left_joins and where to find missing relationship records. It can be expressed in a more simplified.

# bad
Post.left_joins(:author).where(authors: { id: nil })

# good
Post.where.missing(:author)

[koic]

It would be better to mention requiring Rails 6.1 or higher. https://github.com/rails/rails/commit/9bfe89e68ee61d9c8ed71ac98c783a7e47bdb608

[ydah]

Indeed it is, I added the supplement that it is for Rails 6.1 or higher.

[pirj]

@koic Please merge on will.

[koic]

Thanks!