Deprecate Relation#sum with a block.

To perform a sum calculation over the array of elements, use to_a.sum(&block).

Please check the discussion in f9cb645d
for more context.

activerecord/CHANGELOG.md

 ## Rails 4.0.0 (unreleased) ##
 
+*   Deprecate calling `Relation#sum` with a block. To perform a calculation over
+    the array result of the relation, use `to_a.sum(&block)`.
+
+    *Carlos Antonio da Silva*
+
 *   Fix postgresql adapter to handle BC timestamps correctly
 
         HistoryEvent.create!(:name => "something", :occured_at => Date.new(0) - 5.years)

activerecord/lib/active_record/associations.rb

@@ -234,7 +234,7 @@ def association_instance_set(name, association)
     #   others.size                       |   X   |    X     |    X
     #   others.length                     |   X   |    X     |    X
     #   others.count                      |   X   |    X     |    X
-    #   others.sum(args*,&block)          |   X   |    X     |    X
+    #   others.sum(*args)                 |   X   |    X     |    X
     #   others.empty?                     |   X   |    X     |    X
     #   others.clear                      |   X   |    X     |    X
     #   others.delete(other,other,...)    |   X   |    X     |    X

activerecord/lib/active_record/relation/calculations.rb

+require 'active_support/deprecation'
+
 module ActiveRecord
   module Calculations
     # Count the records.
@@ -51,6 +53,10 @@ def maximum(column_name, options = {})
     #   Person.sum('age') # => 4562
     def sum(*args)
       if block_given?
+        ActiveSupport::Deprecation.warn(
+          "Calling #sum with a block is deprecated and will be removed in Rails 4.1. " \
+          "If you want to perform sum calculation over the array of elements, use `to_a.sum(&block)`."
+        )
         self.to_a.sum(*args) {|*block_args| yield(*block_args)}
       else
         calculate(:sum, *args)

activerecord/test/cases/associations/has_many_associations_test.rb

@@ -1658,6 +1658,12 @@ def test_collection_association_with_private_kernel_method
     assert_deprecated { klass.has_many :foo, :counter_sql => 'lol' }
   end
 
+  test "sum calculation with block for array compatibility is deprecated" do
+    assert_deprecated do
+      posts(:welcome).comments.sum { |c| c.id }
+    end
+  end
+
   test "has many associations on new records use null relations" do
     post = Post.new
 

activerecord/test/cases/calculations_test.rb

@@ -389,8 +389,10 @@ def test_sum_with_from_option
         Account.where("credit_limit > 50").from('accounts').sum(:credit_limit)
   end
 
-  def test_sum_array_compatibility
-    assert_equal Account.sum(:credit_limit), Account.sum(&:credit_limit)
+  def test_sum_array_compatibility_deprecation
+    assert_deprecated do
+      assert_equal Account.sum(:credit_limit), Account.sum(&:credit_limit)
+    end
   end
 
   def test_average_with_from_option