11 files changed, 122 insertions, 8 deletions
diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index a3b680f09d..57d135d1aa 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,32 @@
+*   Add `ActiveRecord::Base.suppress` to prevent the receiver from being saved
+    during the given block.
+
+    For example, here's a pattern of creating notifications when new comments
+    are posted. (The notification may in turn trigger an email, a push
+    notification, or just appear in the UI somewhere):
+
+        class Comment < ActiveRecord::Base
+          belongs_to :commentable, polymorphic: true
+          after_create -> { Notification.create! comment: self,
+            recipients: commentable.recipients }
+        end
+
+    That's what you want the bulk of the time. New comment creates a new
+    Notification. But there may well be off cases, like copying a commentable
+    and its comments, where you don't want that. So you'd have a concern
+    something like this:
+
+        module Copyable
+          def copy_to(destination)
+            Notification.suppress do
+              # Copy logic that creates new comments that we do not want triggering
+              # notifications.
+            end
+          end
+        end
+
+    *Michael Ryan*
+
 *   `:time` option added for `#touch`
     Fixes #18905.
 
diff --git a/activerecord/lib/active_record.rb b/activerecord/lib/active_record.rb
index d9d47c3d99..ef14e065ae 100644
--- a/activerecord/lib/active_record.rb
+++ b/activerecord/lib/active_record.rb
@@ -62,6 +62,7 @@ module ActiveRecord
   autoload :Serialization
   autoload :StatementCache
   autoload :Store
+  autoload :Suppressor
   autoload :TableMetadata
   autoload :Timestamp
   autoload :Transactions
diff --git a/activerecord/lib/active_record/base.rb b/activerecord/lib/active_record/base.rb
index 100d3780f6..cc03e37a12 100644
--- a/activerecord/lib/active_record/base.rb
+++ b/activerecord/lib/active_record/base.rb
@@ -313,6 +313,7 @@ module ActiveRecord #:nodoc:
     include Serialization
     include Store
     include SecureToken
+    include Suppressor
   end
 
   ActiveSupport.run_load_hooks(:active_record, Base)
diff --git a/activerecord/lib/active_record/connection_adapters/mysql_adapter.rb b/activerecord/lib/active_record/connection_adapters/mysql_adapter.rb
index 16f50fc594..64985ee933 100644
--- a/activerecord/lib/active_record/connection_adapters/mysql_adapter.rb
+++ b/activerecord/lib/active_record/connection_adapters/mysql_adapter.rb
@@ -328,8 +328,8 @@ module ActiveRecord
 
       def initialize_type_map(m) # :nodoc:
         super
-        m.register_type %r(datetime)i, Fields::DateTime.new
-        m.register_type %r(time)i,     Fields::Time.new
+        register_class_with_precision m, %r(datetime)i, Fields::DateTime
+        register_class_with_precision m, %r(time)i,     Fields::Time
       end
 
       def exec_without_stmt(sql, name = 'SQL') # :nodoc:
diff --git a/activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb b/activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb
index 5a887ea529..b4fe7506f8 100644
--- a/activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb
+++ b/activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb
@@ -477,7 +477,6 @@ module ActiveRecord
           register_class_with_limit m, 'varbit', OID::BitVarying
           m.alias_type 'timestamptz', 'timestamp'
           m.register_type 'date', Type::Date.new
-          m.register_type 'time', Type::Time.new
 
           m.register_type 'money', OID::Money.new
           m.register_type 'bytea', OID::Bytea.new
@@ -503,10 +502,8 @@ module ActiveRecord
           m.alias_type 'lseg', 'varchar'
           m.alias_type 'box', 'varchar'
 
-          m.register_type 'timestamp' do |_, _, sql_type|
-            precision = extract_precision(sql_type)
-            OID::DateTime.new(precision: precision)
-          end
+          register_class_with_precision m, 'time', Type::Time
+          register_class_with_precision m, 'timestamp', OID::DateTime
 
           m.register_type 'numeric' do |_, fmod, sql_type|
             precision = extract_precision(sql_type)
diff --git a/activerecord/lib/active_record/suppressor.rb b/activerecord/lib/active_record/suppressor.rb
new file mode 100644
index 0000000000..b0b86865fd
--- /dev/null
+++ b/activerecord/lib/active_record/suppressor.rb
@@ -0,0 +1,55 @@
+module ActiveRecord
+  # ActiveRecord::Suppressor prevents the receiver from being saved during
+  # a given block.
+  #
+  # For example, here's a pattern of creating notifications when new comments
+  # are posted. (The notification may in turn trigger an email, a push
+  # notification, or just appear in the UI somewhere):
+  #
+  #   class Comment < ActiveRecord::Base
+  #     belongs_to :commentable, polymorphic: true
+  #     after_create -> { Notification.create! comment: self,
+  #       recipients: commentable.recipients }
+  #   end
+  #
+  # That's what you want the bulk of the time. New comment creates a new
+  # Notification. But there may well be off cases, like copying a commentable
+  # and its comments, where you don't want that. So you'd have a concern
+  # something like this:
+  #
+  #   module Copyable
+  #     def copy_to(destination)
+  #       Notification.suppress do
+  #         # Copy logic that creates new comments that we do not want
+  #         # triggering notifications.
+  #       end
+  #     end
+  #   end
+  module Suppressor
+    extend ActiveSupport::Concern
+
+    module ClassMethods
+      def suppress(&block)
+        SuppressorRegistry.suppressed[name] = true
+        yield
+      ensure
+        SuppressorRegistry.suppressed[name] = false
+      end
+    end
+
+    # Ignore saving events if we're in suppression mode.
+    def save!(*args) # :nodoc:
+      SuppressorRegistry.suppressed[self.class.name] ? true : super
+    end
+  end
+
+  class SuppressorRegistry # :nodoc:
+    extend ActiveSupport::PerThreadRegistry
+
+    attr_reader :suppressed
+
+    def initialize
+      @suppressed = {}
+    end
+  end
+end
diff --git a/activerecord/test/cases/helper.rb b/activerecord/test/cases/helper.rb
index f1f927852c..f2ba28a32f 100644
--- a/activerecord/test/cases/helper.rb
+++ b/activerecord/test/cases/helper.rb
@@ -46,7 +46,7 @@ def in_memory_db?
 end
 
 def mysql_56?
-  current_adapter?(:Mysql2Adapter) &&
+  current_adapter?(:MysqlAdapter, :Mysql2Adapter) &&
     ActiveRecord::Base.connection.send(:version).join(".") >= "5.6.0"
 end
 
diff --git a/activerecord/test/cases/suppressor_test.rb b/activerecord/test/cases/suppressor_test.rb
new file mode 100644
index 0000000000..1c449d42fe
--- /dev/null
+++ b/activerecord/test/cases/suppressor_test.rb
@@ -0,0 +1,21 @@
+require 'cases/helper'
+require 'models/notification'
+require 'models/user'
+
+class SuppressorTest < ActiveRecord::TestCase
+  def test_suppresses_creation_of_record_generated_by_callback
+    assert_difference -> { User.count } do
+      assert_no_difference -> { Notification.count } do
+        Notification.suppress { UserWithNotification.create! }
+      end
+    end
+  end
+
+  def test_resumes_saving_after_suppression_complete
+    Notification.suppress { UserWithNotification.create! }
+
+    assert_difference -> { Notification.count } do
+      Notification.create!
+    end
+  end
+end
diff --git a/activerecord/test/models/notification.rb b/activerecord/test/models/notification.rb
new file mode 100644
index 0000000000..b4b4b8f1b6
--- /dev/null
+++ b/activerecord/test/models/notification.rb
@@ -0,0 +1,2 @@
+class Notification < ActiveRecord::Base
+end
diff --git a/activerecord/test/models/user.rb b/activerecord/test/models/user.rb
index 23cd2e0e1c..f5dc93e994 100644
--- a/activerecord/test/models/user.rb
+++ b/activerecord/test/models/user.rb
@@ -2,3 +2,7 @@ class User < ActiveRecord::Base
   has_secure_token
   has_secure_token :auth_token
 end
+
+class UserWithNotification < User
+  after_create -> { Notification.create! message: "A new user has been created." }
+end
diff --git a/activerecord/test/schema/schema.rb b/activerecord/test/schema/schema.rb
index a7d90e3f89..08a16d5c9e 100644
--- a/activerecord/test/schema/schema.rb
+++ b/activerecord/test/schema/schema.rb
@@ -468,6 +468,10 @@ ActiveRecord::Schema.define do
     t.string      :name
   end
 
+  create_table :notifications, force: true do |t|
+    t.string :message
+  end
+
   create_table :numeric_data, force: true do |t|
     t.decimal :bank_balance, precision: 10, scale: 2
     t.decimal :big_bank_balance, precision: 15, scale: 2