Fixed taking precision into count when assigning a value to timestamp attribute

Timestamp column can have less precision than ruby timestamp
In result in how big a fraction of a second can be stored in the
database.

  m = Model.create!
  m.created_at.usec == m.reload.created_at.usec
    # => false
    # due to different seconds precision in Time.now and database column

If the precision is low enough, (mysql default is 0, so it is always low
enough by default) the value changes when model is reloaded from the
database. This patch fixes that issue ensuring that any timestamp
assigned as an attribute is converted to column precision under the
attribute.

6 files changed, 67 insertions, 35 deletions

diff --git a/activemodel/lib/active_model/type/date_time.rb b/activemodel/lib/active_model/type/date_time.rb
index b068cfc672..2f2df4320f 100644
--- a/activemodel/lib/active_model/type/date_time.rb
+++ b/activemodel/lib/active_model/type/date_time.rb
@@ -12,11 +12,11 @@ module ActiveModel
 
       private
 
-      def cast_value(string)
-        return string unless string.is_a?(::String)
-        return if string.empty?
+      def cast_value(value)
+        return apply_seconds_precision(value) unless value.is_a?(::String)
+        return if value.empty?
 
-        fast_string_to_time(string) || fallback_string_to_time(string)
+        fast_string_to_time(value) || fallback_string_to_time(value)
       end
 
       # '0.123456' -> 123456
diff --git a/activemodel/lib/active_model/type/helpers/time_value.rb b/activemodel/lib/active_model/type/helpers/time_value.rb
index 0a0e58654b..63993c0d93 100644
--- a/activemodel/lib/active_model/type/helpers/time_value.rb
+++ b/activemodel/lib/active_model/type/helpers/time_value.rb
@@ -5,11 +5,7 @@ module ActiveModel
     module Helpers
       module TimeValue # :nodoc:
         def serialize(value)
-          if precision && value.respond_to?(:usec)
-            number_of_insignificant_digits = 6 - precision
-            round_power = 10 ** number_of_insignificant_digits
-            value = value.change(usec: value.usec / round_power * round_power)
-          end
+          value = apply_seconds_precision(value)
 
           if value.acts_like?(:time)
             zone_conversion_method = is_utc? ? :getutc : :getlocal
@@ -34,6 +30,13 @@ module ActiveModel
           end
         end
 
+        def apply_seconds_precision(value)
+          return value unless precision && value.respond_to?(:usec)
+          number_of_insignificant_digits = 6 - precision
+          round_power = 10 ** number_of_insignificant_digits
+          value.change(usec: value.usec / round_power * round_power)
+        end
+
         def type_cast_for_schema(value)
           "'#{value.to_s(:db)}'"
         end
diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index 8befdfcdba..1d937fa0b6 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,23 @@
+*   Fixed taking precision into count when assigning a value to timestamp attribute
+
+    Timestamp column can have less precision than ruby timestamp
+    In result in how big a fraction of a second can be stored in the
+    database.
+
+
+      m = Model.create!
+      m.created_at.usec == m.reload.created_at.usec
+        # => false
+        # due to different precision in Time.now and database column
+
+    If the precision is low enough, (mysql default is 0, so it is always low
+    enough by default) the value changes when model is reloaded from the
+    database. This patch fixes that issue ensuring that any timestamp
+    assigned as an attribute is converted to column precision under the
+    attribute.
+
+    *Bogdan Gusiev*
+
 *   Ensure `select` quotes aliased attributes, even when using `from`.
 
     Fixes #21488
diff --git a/activerecord/lib/active_record/connection_adapters/postgresql/oid/date_time.rb b/activerecord/lib/active_record/connection_adapters/postgresql/oid/date_time.rb
index 2c04c46131..424769f765 100644
--- a/activerecord/lib/active_record/connection_adapters/postgresql/oid/date_time.rb
+++ b/activerecord/lib/active_record/connection_adapters/postgresql/oid/date_time.rb
@@ -4,18 +4,14 @@ module ActiveRecord
       module OID # :nodoc:
         class DateTime < Type::DateTime # :nodoc:
           def cast_value(value)
-            if value.is_a?(::String)
-              case value
-              when 'infinity' then ::Float::INFINITY
-              when '-infinity' then -::Float::INFINITY
-              when / BC$/
-                astronomical_year = format("%04d", -value[/^\d+/].to_i + 1)
-                super(value.sub(/ BC$/, "").sub(/^\d+/, astronomical_year))
-              else
-                super
-              end
+            case value
+            when 'infinity' then ::Float::INFINITY
+            when '-infinity' then -::Float::INFINITY
+            when / BC$/
+              astronomical_year = format("%04d", -value[/^\d+/].to_i + 1)
+              super(value.sub(/ BC$/, "").sub(/^\d+/, astronomical_year))
             else
-              value
+              super
             end
           end
         end
diff --git a/activerecord/test/cases/type/date_time_test.rb b/activerecord/test/cases/type/date_time_test.rb
new file mode 100644
index 0000000000..62d3405be1
--- /dev/null
+++ b/activerecord/test/cases/type/date_time_test.rb
@@ -0,0 +1,13 @@
+require "cases/helper"
+require "models/task"
+
+module ActiveRecord
+  module Type
+    class IntegerTest < ActiveRecord::TestCase
+      def test_datetime_seconds_precision_applied_to_timestamp
+        p = Task.create!(starting: ::Time.now)
+        assert_equal p.starting.usec, p.reload.starting.usec
+      end
+    end
+  end
+end
diff --git a/activerecord/test/schema/schema.rb b/activerecord/test/schema/schema.rb
index d318f3a968..ae183ade81 100644
--- a/activerecord/test/schema/schema.rb
+++ b/activerecord/test/schema/schema.rb
@@ -251,10 +251,10 @@ ActiveRecord::Schema.define do
   create_table :developers, force: true do |t|
     t.string   :name
     t.integer  :salary, default: 70000
-    t.datetime :created_at
-    t.datetime :updated_at
-    t.datetime :created_on
-    t.datetime :updated_on
+    t.datetime :created_at, precision: 6
+    t.datetime :updated_at, precision: 6
+    t.datetime :created_on, precision: 6
+    t.datetime :updated_on, precision: 6
   end
 
   create_table :developers_projects, force: true, id: false do |t|
@@ -353,7 +353,7 @@ ActiveRecord::Schema.define do
 
   create_table :invoices, force: true do |t|
     t.integer :balance
-    t.datetime :updated_at
+    t.datetime :updated_at, precision: 6
   end
 
   create_table :iris, force: true do |t|
@@ -503,7 +503,7 @@ ActiveRecord::Schema.define do
 
   create_table :owners, primary_key: :owner_id, force: true do |t|
     t.string :name
-    t.column :updated_at, :datetime
+    t.column :updated_at, :datetime, precision: 6
     t.column :happy_at,   :datetime
     t.string :essay_id
   end
@@ -521,10 +521,10 @@ ActiveRecord::Schema.define do
     t.column :color, :string
     t.column :parrot_sti_class, :string
     t.column :killer_id, :integer
-    t.column :created_at, :datetime
-    t.column :created_on, :datetime
-    t.column :updated_at, :datetime
-    t.column :updated_on, :datetime
+    t.column :created_at, :datetime, precision: 0
+    t.column :created_on, :datetime, precision: 0
+    t.column :updated_at, :datetime, precision: 0
+    t.column :updated_on, :datetime, precision: 0
   end
 
   create_table :parrots_pirates, id: false, force: true do |t|
@@ -567,15 +567,15 @@ ActiveRecord::Schema.define do
   create_table :pets, primary_key: :pet_id, force: true do |t|
     t.string :name
     t.integer :owner_id, :integer
-    t.timestamps null: false
+    t.timestamps null: false, precision: 6
   end
 
   create_table :pirates, force: true do |t|
     t.column :catchphrase, :string
     t.column :parrot_id, :integer
     t.integer :non_validated_parrot_id
-    t.column :created_on, :datetime
-    t.column :updated_on, :datetime
+    t.column :created_on, :datetime, precision: 6
+    t.column :updated_on, :datetime, precision: 6
   end
 
   create_table :posts, force: true do |t|
@@ -685,7 +685,7 @@ ActiveRecord::Schema.define do
   create_table :ship_parts, force: true do |t|
     t.string :name
     t.integer :ship_id
-    t.datetime :updated_at
+    t.datetime :updated_at, precision: 6
   end
 
   create_table :prisoners, force: true do |t|
@@ -778,7 +778,7 @@ ActiveRecord::Schema.define do
     t.string   :parent_title
     t.string   :type
     t.string   :group
-    t.timestamps null: true
+    t.timestamps null: true, precision: 6
   end
 
   create_table :toys, primary_key: :toy_id, force: true do |t|