3 files changed, 37 insertions, 2 deletions
diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index c78e602af7..8356b49bec 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,5 +1,25 @@
 ## Rails 4.0.0 (unreleased) ##
 
+*   Allow Relation#merge to take a proc.
+
+    This was requested by DHH to allow creating of one's own custom
+    association macros.
+
+    For example:
+
+        module Commentable
+          def has_many_comments(extra)
+            has_many :comments, -> { where(:foo).merge(extra) }
+          end
+        end
+
+        class Post < ActiveRecord::Base
+          extend Commentable
+          has_many_comments -> { where(:bar) }
+        end
+
+    *Jon Leighton*
+
 *   Add CollectionProxy#scope
 
     This can be used to get a Relation from an association.
diff --git a/activerecord/lib/active_record/relation/spawn_methods.rb b/activerecord/lib/active_record/relation/spawn_methods.rb
index dcd4c78eb7..5394c1b28b 100644
--- a/activerecord/lib/active_record/relation/spawn_methods.rb
+++ b/activerecord/lib/active_record/relation/spawn_methods.rb
@@ -23,6 +23,13 @@ module ActiveRecord
     #   # Returns the intersection of all published posts with the 5 most recently created posts.
     #   # (This is just an example. You'd probably want to do this with a single query!)
     #
+    # Procs will be evaluated by merge:
+    #
+    #   Post.where(published: true).merge(-> { joins(:comments) })
+    #   # => Post.where(published: true).joins(:comments)
+    #
+    # This is mainly intended for sharing common conditions between multiple associations.
+    #
     def merge(other)
       if other.is_a?(Array)
         to_a & other
@@ -35,8 +42,12 @@ module ActiveRecord
 
     # Like #merge, but applies changes in place.
     def merge!(other)
-      klass = other.is_a?(Hash) ? Relation::HashMerger : Relation::Merger
-      klass.new(self, other).merge
+      if !other.is_a?(Relation) && other.respond_to?(:to_proc)
+        instance_exec(&other)
+      else
+        klass = other.is_a?(Hash) ? Relation::HashMerger : Relation::Merger
+        klass.new(self, other).merge
+      end
     end
 
     # Removes from the query the condition(s) specified in +skips+.
diff --git a/activerecord/test/cases/relation_test.rb b/activerecord/test/cases/relation_test.rb
index 034339e413..5fb54b1ca1 100644
--- a/activerecord/test/cases/relation_test.rb
+++ b/activerecord/test/cases/relation_test.rb
@@ -247,5 +247,9 @@ module ActiveRecord
       assert relation.merge!(where: :foo).equal?(relation)
       assert_equal [:foo], relation.where_values
     end
+
+    test 'merge with a proc' do
+      assert_equal [:foo], relation.merge(-> { where(:foo) }).where_values
+    end
   end
 end