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

1 files changed, 13 insertions, 2 deletions

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+.