Implement Relation#new

activerecord/lib/active_record/relation.rb

@@ -15,6 +15,10 @@ def initialize(klass, relation)
       @loaded, @readonly = false
     end
 
+    def new(*args, &block)
+      @klass.send(:with_scope, :create => create_scope) { @klass.new(*args, &block) }
+    end
+
     def merge(r)
       raise ArgumentError, "Cannot merge a #{r.klass.name} relation with #{@klass.name} relation" if r.klass != @klass
 
@@ -138,7 +142,7 @@ def reload
     end
 
     def reset
-      @first = @last = nil
+      @first = @last = @create_scope = nil
       @records = []
       self
     end
@@ -181,6 +185,13 @@ def method_missing(method, *args, &block)
       end
     end
 
+    def create_scope
+      @create_scope ||= wheres.inject({}) do |hash, where|
+        hash[where.operand1.name] = where.operand2.value if where.is_a?(Arel::Predicates::Equality)
+        hash
+      end
+    end
+
     def where_clause(join_string = " AND ")
       @relation.send(:where_clauses).join(join_string)
     end

activerecord/test/cases/relations_test.rb

@@ -477,4 +477,20 @@ def test_many_with_limits
     assert posts.many?
     assert ! posts.limit(1).many?
   end
+
+  def test_build
+    posts = Post.scoped
+
+    post = posts.new
+    assert_kind_of Post, post
+  end
+
+  def test_scoped_build
+    posts = Post.where(:title => 'You told a lie')
+
+    post = posts.new
+    assert_kind_of Post, post
+    assert_equal 'You told a lie', post.title
+  end
+
 end