Prior to this commit if you defined a bidirectional relationship
between two models with destroy dependencies on both sides, a call to
`destroy` would result in an infinite callback loop.
Take the following relationship.
class Content < ActiveRecord::Base
has_one :content_position, dependent: :destroy
end
class ContentPosition < ActiveRecord::Base
belongs_to :content, dependent: :destroy
end
Calling `Content#destroy` or `ContentPosition#destroy` would result in
an infinite callback loop.
This commit changes the behaviour of `ActiveRecord::Callbacks#destroy`
so that it guards against subsequent callbacks.
Thanks to @zetter for demonstrating the issue with failing tests[1].
[1] rails#13609
