Merge pull request #4805 from xuanxu/none_and_null_object_pattern

Added `none` query method to return zero records.

3 files changed, 44 insertions, 1 deletions

diff --git a/activerecord/lib/active_record/null_relation.rb b/activerecord/lib/active_record/null_relation.rb
new file mode 100644
index 0000000000..60c37ac2b7
--- /dev/null
+++ b/activerecord/lib/active_record/null_relation.rb
@@ -0,0 +1,10 @@
+# -*- coding: utf-8 -*-
+
+module ActiveRecord
+  # = Active Record Null Relation
+  class NullRelation < Relation
+    def exec_queries
+      @records = []
+    end
+  end
+end
\ No newline at end of file
diff --git a/activerecord/lib/active_record/querying.rb b/activerecord/lib/active_record/querying.rb
index 34ba480d83..5945b05190 100644
--- a/activerecord/lib/active_record/querying.rb
+++ b/activerecord/lib/active_record/querying.rb
@@ -8,7 +8,7 @@ module ActiveRecord
     delegate :find_each, :find_in_batches, :to => :scoped
     delegate :select, :group, :order, :except, :reorder, :limit, :offset, :joins,
              :where, :preload, :eager_load, :includes, :from, :lock, :readonly,
-             :having, :create_with, :uniq, :references, :to => :scoped
+             :having, :create_with, :uniq, :references, :none, :to => :scoped
     delegate :count, :average, :minimum, :maximum, :sum, :calculate, :pluck, :to => :scoped
 
     # Executes a custom SQL query against your database and returns all the results. The results will
diff --git a/activerecord/lib/active_record/relation/query_methods.rb b/activerecord/lib/active_record/relation/query_methods.rb
index a8ae7208fc..6a28d7b155 100644
--- a/activerecord/lib/active_record/relation/query_methods.rb
+++ b/activerecord/lib/active_record/relation/query_methods.rb
@@ -196,6 +196,39 @@ module ActiveRecord
       relation
     end
 
+    # Returns a chainable relation with zero records, specifically an
+    # instance of the NullRelation class.
+    #
+    # The returned NullRelation inherits from Relation and implements the
+    # Null Object pattern so it is an object with defined null behavior:
+    # it always returns an empty array of records and avoids any database query.
+    #
+    # Any subsequent condition chained to the returned relation will continue
+    # generating an empty relation and will not fire any query to the database.
+    #
+    # Used in cases where is needed a method or a scope that could return zero
+    # results but the response has to be chainable.
+    #
+    # For example:
+    #
+    #   @posts = current_user.visible_posts.where(:name => params[:name])
+    #   # => the visible_post method response has to be a chainable Relation
+    #
+    #   def visible_posts
+    #     case role
+    #     if 'Country Manager'
+    #       Post.where(:country => country)
+    #     if 'Reviewer'
+    #       Post.published
+    #     if 'Bad User'
+    #       Post.none # => returning [] instead breaks the previous code
+    #     end
+    #   end
+    #
+    def none
+      NullRelation.new(@klass, @table)
+    end
+
     def readonly(value = true)
       relation = clone
       relation.readonly_value = value