Add Relation#includes to be an equivalent of current finder option :include

activerecord/lib/active_record/associations/association_collection.rb

@@ -21,7 +21,7 @@ def initialize(owner, reflection)
         construct_sql
       end
 
-      delegate :group, :order, :limit, :joins, :where, :preload, :eager_load, :from, :lock, :readonly, :having, :to => :scoped
+      delegate :group, :order, :limit, :joins, :where, :preload, :eager_load, :includes, :from, :lock, :readonly, :having, :to => :scoped
 
       def select(select = nil, &block)
         if block_given?

activerecord/lib/active_record/base.rb

@@ -655,7 +655,7 @@ def find(*args)
         end
       end
 
-      delegate :select, :group, :order, :limit, :joins, :where, :preload, :eager_load, :from, :lock, :readonly, :having, :to => :scoped
+      delegate :select, :group, :order, :limit, :joins, :where, :preload, :eager_load, :includes, :from, :lock, :readonly, :having, :to => :scoped
 
       # A convenience wrapper for <tt>find(:first, *args)</tt>. You can pass in all the
       # same arguments to this method as you can to <tt>find(:first)</tt>.

activerecord/lib/active_record/relation.rb

@@ -5,13 +5,15 @@ class Relation
     delegate :to_sql, :to => :relation
     delegate :length, :collect, :map, :each, :all?, :to => :to_a
 
-    attr_reader :relation, :klass, :preload_associations, :eager_load_associations
-    attr_writer :readonly, :preload_associations, :eager_load_associations, :table
+    attr_reader :relation, :klass
+    attr_writer :readonly, :table
+    attr_accessor :preload_associations, :eager_load_associations, :include_associations
 
     def initialize(klass, relation)
       @klass, @relation = klass, relation
       @preload_associations = []
       @eager_load_associations = []
+      @include_associations = []
       @loaded, @readonly = false
     end
 
@@ -30,7 +32,7 @@ def create!(*args, &block)
     def merge(r)
       raise ArgumentError, "Cannot merge a #{r.klass.name} relation with #{@klass.name} relation" if r.klass != @klass
 
-      merged_relation = spawn(table).eager_load(r.eager_load_associations).preload(r.preload_associations)
+      merged_relation = spawn(table).eager_load(r.eager_load_associations).preload(r.preload_associations).includes(r.include_associations)
       merged_relation.readonly = r.readonly
 
       [self.relation, r.relation].each do |arel|
@@ -74,7 +76,9 @@ def respond_to?(method, include_private = false)
     def to_a
       return @records if loaded?
 
-      @records = if @eager_load_associations.any?
+      find_with_associations = @eager_load_associations.any?
+
+      @records = if find_with_associations
         begin
           @klass.send(:find_with_associations, {
             :select => @relation.send(:select_clauses).join(', '),
@@ -94,7 +98,10 @@ def to_a
         @klass.find_by_sql(@relation.to_sql)
       end
 
-      @preload_associations.each {|associations| @klass.send(:preload_associations, @records, associations) }
+      preload = @preload_associations
+      preload +=  @include_associations unless find_with_associations
+      preload.each {|associations| @klass.send(:preload_associations, @records, associations) } 
+
       @records.each { |record| record.readonly! } if @readonly
 
       @loaded = true
@@ -160,6 +167,7 @@ def spawn(relation = @relation)
       relation.readonly = @readonly
       relation.preload_associations = @preload_associations
       relation.eager_load_associations = @eager_load_associations
+      relation.include_associations = @include_associations
       relation.table = table
       relation
     end

activerecord/lib/active_record/relation/query_methods.rb

@@ -5,6 +5,10 @@ def preload(*associations)
       spawn.tap {|r| r.preload_associations += Array.wrap(associations) }
     end
 
+    def includes(*associations)
+      spawn.tap {|r| r.include_associations += Array.wrap(associations) }
+    end
+
     def eager_load(*associations)
       spawn.tap {|r| r.eager_load_associations += Array.wrap(associations) }
     end

activerecord/test/cases/relations_test.rb

@@ -178,7 +178,7 @@ def test_eager_association_loading_of_stis_with_multiple_references
     end
   end
 
-  def test_find_with_included_associations
+  def test_find_with_preloaded_associations
     assert_queries(2) do
       posts = Post.preload(:comments)
       assert posts.first.comments.first
@@ -206,6 +206,29 @@ def test_find_with_included_associations
     end
   end
 
+  def test_find_with_included_associations
+    assert_queries(2) do
+      posts = Post.includes(:comments)
+      assert posts.first.comments.first
+    end
+
+    assert_queries(2) do
+      posts = Post.scoped.includes(:comments)
+      assert posts.first.comments.first
+    end
+
+    assert_queries(2) do
+      posts = Post.includes(:author)
+      assert posts.first.author
+    end
+
+    assert_queries(3) do
+      posts = Post.includes(:author, :comments).to_a
+      assert posts.first.author
+      assert posts.first.comments.first
+    end
+  end
+
   def test_default_scope_with_conditions_string
     assert_equal Developer.find_all_by_name('David').map(&:id).sort, DeveloperCalledDavid.scoped.map(&:id).sort
     assert_equal nil, DeveloperCalledDavid.create!.name