Allow ActiveRecord::Relation#update to run on result of a relation with callbacks and validations

- Right now, there is no method to update multiple records with
  validations and callbacks.
- Changed the behavior of existing `update` method so that when `id`
  attribute is not given and the method is called on an `Relation`
  object, it will execute update for every record of the `Relation` and
  will run validations and callbacks for every record.
- Added test case for validating that the callbacks run when `update` is
  called on a `Relation`.
- Changed test_create_columns_not_equal_attributes test from
  persistence_test to include author_name column on topics table as it
  it used in before_update callback.
- This change introduces performance issues when a large number of
  records are to be updated because it runs UPDATE query for every
  record of the result. The `update_all` method can be used in that case
  if callbacks are not required because it will only run single UPDATE
  for all the records.

1 files changed, 13 insertions, 1 deletions

diff --git a/activerecord/lib/active_record/relation.rb b/activerecord/lib/active_record/relation.rb
index daafb0b645..5ca28033bf 100644
--- a/activerecord/lib/active_record/relation.rb
+++ b/activerecord/lib/active_record/relation.rb
@@ -362,9 +362,21 @@ module ActiveRecord
     #   # Updates multiple records
     #   people = { 1 => { "first_name" => "David" }, 2 => { "first_name" => "Jeremy" } }
     #   Person.update(people.keys, people.values)
-    def update(id, attributes)
+    #
+    #   # Updates multiple records from the result of a relation
+    #   people = Person.where(group: 'expert')
+    #   people.update(group: 'masters')
+    #
+    #   Note: Updating a large number of records will run a
+    #   UPDATE query for each record, which may cause a performance
+    #   issue. So if it is not needed to run callbacks for each update, it is
+    #   preferred to use <tt>update_all</tt> for updating all records using
+    #   a single query.
+    def update(id = :all, attributes)
       if id.is_a?(Array)
         id.map.with_index { |one_id, idx| update(one_id, attributes[idx]) }
+      elsif id == :all
+        to_a.each { |record| record.update(attributes) }
       else
         object = find(id)
         object.update(attributes)