Change behavior of count(:limit => x, :offset => y) to limit/offset before counting.

activerecord/lib/active_record/relation/calculations.rb

@@ -183,10 +183,13 @@ module ActiveRecord
       end
     end
 
-    def aggregate_column(column_name)
+    def aggregate_column(column_name, subquery_alias = nil)
       if @klass.column_names.include?(column_name.to_s)
-        Arel::Attribute.new(@klass.unscoped.table, column_name)
+        Arel::Attribute.new(subquery_alias || @klass.unscoped.table, column_name)
       else
+        if subquery_alias && (split_name = column_name.to_s.split(".")).length > 1
+          column_name = split_name.last
+        end
         Arel.sql(column_name == :all ? "*" : column_name.to_s)
       end
     end
@@ -196,24 +199,22 @@ module ActiveRecord
     end
 
     def execute_simple_calculation(operation, column_name, distinct) #:nodoc:
-      column = aggregate_column(column_name)
-
       # Postgresql doesn't like ORDER BY when there are no GROUP BY
       relation = except(:order)
-      select_value = operation_over_aggregate_column(column, operation, distinct)
 
-      relation.select_values = [select_value]
+      if operation == "count" && (relation.limit_value || relation.offset_value)
+        # Shortcut when limit is zero.
+        return 0 if relation.limit_value == 0
 
-      query_builder = relation.arel
+        query_builder = build_count_subquery(relation, column_name, distinct)
+      else
+        column = aggregate_column(column_name)
 
-      if operation == "count"
-        limit  = relation.limit_value
-        offset = relation.offset_value
+        select_value = operation_over_aggregate_column(column, operation, distinct)
 
-        unless limit && offset
-          query_builder.limit  = nil
-          query_builder.offset = nil
-        end
+        relation.select_values = [select_value]
+
+        query_builder = relation.arel
       end
 
       type_cast_calculated_value(@klass.connection.select_value(query_builder.to_sql), column_for(column_name), operation)
@@ -312,5 +313,16 @@ module ActiveRecord
         select if select !~ /(,|\*)/
       end
     end
+
+    def build_count_subquery(relation, column_name, distinct)
+      # Arel doesn't do subqueries
+      subquery_alias = arel_table.alias("subquery_for_count")
+      aliased_column = aggregate_column(column_name, subquery_alias)
+      select_value = operation_over_aggregate_column(aliased_column, 'count', distinct)
+
+      relation.select_values = [(column_name == :all ? 1 : aggregate_column(column_name))]
+      subquery_sql = "(#{relation.arel.to_sql}) #{subquery_alias.name}"
+      subquery_alias.relation.select_manager.project(select_value).from(subquery_sql)
+    end
   end
 end

activerecord/test/cases/calculations_test.rb

@@ -65,7 +65,7 @@ class CalculationsTest < ActiveRecord::TestCase
     c = Account.sum(:credit_limit, :group => :firm_id)
     [1,6,2].each { |firm_id| assert c.keys.include?(firm_id) }
   end
-  
+
   def test_should_group_by_multiple_fields
     c = Account.count(:all, :group => ['firm_id', :credit_limit])
     [ [nil, 50], [1, 50], [6, 50], [6, 55], [9, 53], [2, 60] ].each { |firm_and_limit| assert c.keys.include?(firm_and_limit) }
@@ -109,6 +109,35 @@ class CalculationsTest < ActiveRecord::TestCase
     assert_equal [2, 6], c.keys.compact
   end
 
+  def test_limit_should_apply_before_count
+    accounts = Account.limit(3).where('firm_id IS NOT NULL')
+
+    assert_equal 3, accounts.count(:firm_id)
+    assert_equal 3, accounts.select(:firm_id).count
+  end
+
+  def test_count_should_shortcut_with_limit_zero
+    accounts = Account.limit(0)
+
+    assert_no_queries { assert_equal 0, accounts.count }
+  end
+
+  def test_limit_is_kept
+    return if current_adapter?(:OracleAdapter)
+
+    queries = assert_sql { Account.limit(1).count }
+    assert_equal 1, queries.length
+    assert_match(/LIMIT/, queries.first)
+  end
+
+  def test_offset_is_kept
+    return if current_adapter?(:OracleAdapter)
+
+    queries = assert_sql { Account.offset(1).count }
+    assert_equal 1, queries.length
+    assert_match(/OFFSET/, queries.first)
+  end
+
   def test_limit_with_offset_is_kept
     return if current_adapter?(:OracleAdapter)
 
@@ -118,20 +147,6 @@ class CalculationsTest < ActiveRecord::TestCase
     assert_match(/OFFSET/, queries.first)
   end
 
-  def test_offset_without_limit_removes_offset
-    queries = assert_sql { Account.offset(1).count }
-    assert_equal 1, queries.length
-    assert_no_match(/LIMIT/, queries.first)
-    assert_no_match(/OFFSET/, queries.first)
-  end
-
-  def test_limit_without_offset_removes_limit
-    queries = assert_sql { Account.limit(1).count }
-    assert_equal 1, queries.length
-    assert_no_match(/LIMIT/, queries.first)
-    assert_no_match(/OFFSET/, queries.first)
-  end
-
   def test_no_limit_no_offset
     queries = assert_sql { Account.count }
     assert_equal 1, queries.length

activerecord/test/cases/relations_test.rb

@@ -666,6 +666,34 @@ class RelationTest < ActiveRecord::TestCase
     assert_no_queries { assert_equal 5, best_posts.size }
   end
 
+  def test_size_with_limit
+    posts = Post.limit(6)
+
+    assert_queries(1) { assert_equal 6, posts.size }
+    assert ! posts.loaded?
+
+    best_posts = posts.where(:comments_count => 0)
+    best_posts.to_a # force load
+    assert_no_queries { assert_equal 5, best_posts.size }
+  end
+
+  def test_size_with_zero_limit
+    posts = Post.limit(0)
+
+    assert_no_queries { assert_equal 0, posts.size }
+    assert ! posts.loaded?
+
+    posts.to_a # force load
+    assert_no_queries { assert_equal 0, posts.size }
+  end
+
+  def test_empty_with_zero_limit
+    posts = Post.limit(0)
+
+    assert_no_queries { assert_equal true, posts.empty? }
+    assert ! posts.loaded?
+  end
+
   def test_count_complex_chained_relations
     posts = Post.select('comments_count').where('id is not null').group("author_id").where("comments_count > 0")