Fix that a collection proxy could be cached before the save of the owner, resulting in an invalid proxy lacking the owner’s id.

Absent this fix calls like: owner.association.update_all to behave unexpectedly because they try to act on association objects where
owner_id is null.

more evidence here: https://gist.github.com/Empact/5865555
```
Active Record 3.2.13
-- create_table(:firms, {:force=>true})
   -> 0.1371s
-- create_table(:clients, {:force=>true})
   -> 0.0005s
1 clients. 1 expected.
1 clients updated. 1 expected.
```

```
Active Record 4.0.0
-- create_table(:firms, {:force=>true})
   -> 0.1606s
-- create_table(:clients, {:force=>true})
   -> 0.0004s
1 clients. 1 expected.
0 clients updated. 1 expected.
```

1 files changed, 7 insertions, 1 deletions

diff --git a/activerecord/lib/active_record/associations/collection_association.rb b/activerecord/lib/active_record/associations/collection_association.rb
index 1836ff0910..c5dff7ee1a 100644
--- a/activerecord/lib/active_record/associations/collection_association.rb
+++ b/activerecord/lib/active_record/associations/collection_association.rb
@@ -33,7 +33,13 @@ module ActiveRecord
           reload
         end
 
-        @proxy ||= CollectionProxy.create(klass, self)
+        if owner.new_record?
+          # Cache the proxy separately before the owner has an id
+          # or else a post-save proxy will still lack the id
+          @new_record_proxy ||= CollectionProxy.create(klass, self)
+        else
+          @proxy ||= CollectionProxy.create(klass, self)
+        end
       end
 
       # Implements the writer method, e.g. foo.items= for Foo.has_many :items