Forward ActiveRecord::Relation#count to Enumerable#count if block given

activerecord/lib/active_record/associations/collection_association.rb

@@ -246,9 +246,12 @@ def destroy_all
         end
       end
 
-      # Count all records using SQL.  Construct options and pass them with
-      # scope to the target class's +count+.
+      # Returns the number of records. If no arguments are given, it counts all
+      # columns using SQL. If one argument is given, it counts only the passed
+      # column using SQL. If a block is given, it counts the number of records
+      # yielding a true value.
       def count(column_name = nil)
+        return super if block_given?
         relation = scope
         if association_scope.distinct_value
           # This is needed because 'SELECT count(DISTINCT *)..' is not valid SQL.

activerecord/lib/active_record/associations/collection_proxy.rb

@@ -715,12 +715,13 @@ def distinct
       end
       alias uniq distinct
 
-      # Count all records using SQL.
+      # Count all records.
       #
       #   class Person < ActiveRecord::Base
       #     has_many :pets
       #   end
       #
+      #   # This will perform the count using SQL.
       #   person.pets.count # => 3
       #   person.pets
       #   # => [
@@ -728,8 +729,13 @@ def distinct
       #   #       #<Pet id: 2, name: "Spook", person_id: 1>,
       #   #       #<Pet id: 3, name: "Choo-Choo", person_id: 1>
       #   #    ]
-      def count(column_name = nil)
-        @association.count(column_name)
+      #
+      # Passing a block will select all of a person's pets in SQL and then
+      # perform the count using Ruby.
+      #
+      #   person.pets.count { |pet| pet.name.include?('-') } # => 2
+      def count(column_name = nil, &block)
+        @association.count(column_name, &block)
       end
 
       # Returns the size of the collection. If the collection hasn't been loaded,

activerecord/lib/active_record/relation/calculations.rb

@@ -37,7 +37,11 @@ module Calculations
     # Note: not all valid {Relation#select}[rdoc-ref:QueryMethods#select] expressions are valid #count expressions. The specifics differ
     # between databases. In invalid cases, an error from the database is thrown.
     def count(column_name = nil)
-      calculate(:count, column_name)
+      if block_given?
+        to_a.count { |*block_args| yield(*block_args) }
+      else
+        calculate(:count, column_name)
+      end
     end
 
     # Calculates the average value on a given column. Returns +nil+ if there's

activerecord/test/cases/calculations_test.rb

@@ -482,6 +482,10 @@ def test_count_with_where_and_order
     assert_equal 1, Account.where(firm_name: '37signals').order(:firm_name).reverse_order.count
   end
 
+  def test_count_with_block
+    assert_equal 4, Account.count { |account| account.credit_limit.modulo(10).zero? }
+  end
+
   def test_should_sum_expression
     # Oracle adapter returns floating point value 636.0 after SUM
     if current_adapter?(:OracleAdapter)

activerecord/test/cases/relations_test.rb

@@ -1086,6 +1086,11 @@ def test_count
     assert_equal 9, posts.where(:comments_count => 0).count
   end
 
+  def test_count_with_block
+    posts = Post.all
+    assert_equal 10, posts.count { |p| p.comments_count.even? }
+  end
+
   def test_count_on_association_relation
     author = Author.last
     another_author = Author.first