rails/rails — pull request #7661 by ernie (2012-09-17 01:27:37+00:00)
Title: Fix collection= on hm:t join models when unsaved

If assigning to a has_many :through collection against an unsaved
object using the collection=[] syntax, the join models
were not properly created, previously.

This bug is present in the current 3.2.8 release, as well.