Nullify the relation at a more general level.

This allows us to avoid hacks like the "return 0 if owner.new_record?"
in #count (which this commit removes).

Also, the relevant foreign key may actually be present even on a new
owner record, in which case we *don't* want a null relation. This logic
is encapsulated in the #null_scope? method.

We also need to make sure that the CollectionProxy is not 'infected'
with the NullRelation module, or else the methods from there will
override the definitions in CollectionProxy, leading to incorrect
results. Hence the nullify: false option to CollectionAssociation#scope.
(This feels a bit nasty but I can't think of a better way.)

activerecord/lib/active_record/associations/collection_association.rb

@@ -174,8 +174,6 @@ def sum(*args)
       # association, it will be used for the query. Otherwise, construct options and pass them with
       # scope to the target class's +count+.
       def count(column_name = nil, count_options = {})
-        return 0 if owner.new_record?
-
         column_name, count_options = nil, column_name if column_name.is_a?(Hash)
 
         if options[:counter_sql] || options[:finder_sql]
@@ -366,6 +364,16 @@ def add_to_target(record)
         record
       end
 
+      def scope(opts = {})
+        scope = super()
+        scope.none! if opts.fetch(:nullify, true) && null_scope?
+        scope
+      end
+
+      def null_scope?
+        owner.new_record? && !foreign_key_present?
+      end
+
       private
 
         def custom_counter_sql

activerecord/lib/active_record/associations/collection_proxy.rb

@@ -31,7 +31,7 @@ class CollectionProxy < Relation
       def initialize(association) #:nodoc:
         @association = association
         super association.klass, association.klass.arel_table
-        merge! association.scope
+        merge! association.scope(nullify: false)
       end
 
       def target
@@ -835,9 +835,8 @@ def scoping
       # Returns a <tt>Relation</tt> object for the records in this association
       def scope
         association = @association
-        scope = @association.scope
-        scope.none! if @association.owner.new_record?
-        scope.extending! do
+
+        @association.scope.extending! do
           define_method(:proxy_association) { association }
         end
       end

activerecord/lib/active_record/null_relation.rb

@@ -46,7 +46,7 @@ def where_values_hash
       {}
     end
 
-    def count
+    def count(*)
       0
     end