destroys association records before saving/inserting new association records

fixes bug introduced by  #3329
These are the conditions necessary to reproduce the bug:
- For an association, autosave => true.
- An association record is being destroyed
- A new association record is being created.
- There is a unique index one of the association's fields.
- The record being created has the same value as the record being
destroyed on the indexed field.

Before, the deletion of records was postponed until after all
insertions/saves.  Therefore the new record with the identical value in
the indexed field caused a non-unique value error to be thrown at the database
level.

With this fix, the deletions happen first, before the insertions/saves.
Therefore the record with the duplicate value is gone from the database
before the new record is created, thereby avoiding the non-uniuqe value
error.

1 files changed, 8 insertions, 9 deletions

diff --git a/activerecord/lib/active_record/autosave_association.rb b/activerecord/lib/active_record/autosave_association.rb
index b0bd78ad46..87d4daa6d9 100644
--- a/activerecord/lib/active_record/autosave_association.rb
+++ b/activerecord/lib/active_record/autosave_association.rb
@@ -335,15 +335,18 @@ module ActiveRecord
         autosave = reflection.options[:autosave]
 
         if records = associated_records_to_validate_or_save(association, @new_record_before_save, autosave)
-          records_to_destroy = []
+
+          if autosave
+            records_to_destroy = records.select(&:marked_for_destruction?)
+            records_to_destroy.each { |record| association.destroy(record) }
+            records -= records_to_destroy
+          end
+
           records.each do |record|
-            next if record.destroyed?
 
             saved = true
 
-            if autosave && record.marked_for_destruction?
-              records_to_destroy << record
-            elsif autosave != false && (@new_record_before_save || record.new_record?)
+            if autosave != false && (@new_record_before_save || record.new_record?)
               if autosave
                 saved = association.insert_record(record, false)
               else
@@ -355,10 +358,6 @@ module ActiveRecord
 
             raise ActiveRecord::Rollback unless saved
           end
-
-          records_to_destroy.each do |record|
-            association.destroy(record)
-          end
         end
 
         # reconstruct the scope now that we know the owner's id