Correctly dump native timestamp types for MySQL

The native timestamp type in MySQL is different from datetime type.
Internal representation of the timestamp type is UNIX time, This means
that timestamp columns are affected by time zone.

```
> SET time_zone = '+00:00';
Query OK, 0 rows affected (0.00 sec)

> INSERT INTO time_with_zone(ts,dt) VALUES (NOW(),NOW());
Query OK, 1 row affected (0.02 sec)

> SELECT * FROM time_with_zone;
+---------------------+---------------------+
| ts                  | dt                  |
+---------------------+---------------------+
| 2016-02-07 22:11:44 | 2016-02-07 22:11:44 |
+---------------------+---------------------+
1 row in set (0.00 sec)

> SET time_zone = '-08:00';
Query OK, 0 rows affected (0.00 sec)

> SELECT * FROM time_with_zone;
+---------------------+---------------------+
| ts                  | dt                  |
+---------------------+---------------------+
| 2016-02-07 14:11:44 | 2016-02-07 22:11:44 |
+---------------------+---------------------+
1 row in set (0.00 sec)
```

activerecord/CHANGELOG.md

@@ -1,3 +1,36 @@
+*   Correctly dump native timestamp types for MySQL.
+
+    The native timestamp type in MySQL is different from datetime type.
+    Internal representation of the timestamp type is UNIX time, This means
+    that timestamp columns are affected by time zone.
+
+        > SET time_zone = '+00:00';
+        Query OK, 0 rows affected (0.00 sec)
+
+        > INSERT INTO time_with_zone(ts,dt) VALUES (NOW(),NOW());
+        Query OK, 1 row affected (0.02 sec)
+
+        > SELECT * FROM time_with_zone;
+        +---------------------+---------------------+
+        | ts                  | dt                  |
+        +---------------------+---------------------+
+        | 2016-02-07 22:11:44 | 2016-02-07 22:11:44 |
+        +---------------------+---------------------+
+        1 row in set (0.00 sec)
+
+        > SET time_zone = '-08:00';
+        Query OK, 0 rows affected (0.00 sec)
+
+        > SELECT * FROM time_with_zone;
+        +---------------------+---------------------+
+        | ts                  | dt                  |
+        +---------------------+---------------------+
+        | 2016-02-07 14:11:44 | 2016-02-07 22:11:44 |
+        +---------------------+---------------------+
+        1 row in set (0.00 sec)
+
+    *Ryuta Kamizono*
+
 *   All integer-like PKs are autoincrement unless they have an explicit default.
 
     *Matthew Draper*

activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb

@@ -1071,7 +1071,7 @@ module ActiveRecord
               raise ArgumentError, "Error adding decimal column: precision cannot be empty if scale is specified"
             end
 
-          elsif [:datetime, :time, :interval].include?(type) && precision ||= native[:precision]
+          elsif [:datetime, :timestamp, :time, :interval].include?(type) && precision ||= native[:precision]
             if (0..6) === precision
               column_type_sql << "(#{precision})"
             else

activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb

@@ -46,6 +46,7 @@ module ActiveRecord
         float:       { name: "float" },
         decimal:     { name: "decimal" },
         datetime:    { name: "datetime" },
+        timestamp:   { name: "timestamp" },
         time:        { name: "time" },
         date:        { name: "date" },
         binary:      { name: "blob", limit: 65535 },
@@ -708,7 +709,7 @@ module ActiveRecord
         end
 
         def extract_precision(sql_type)
-          if /time/.match?(sql_type)
+          if /\A(?:date)?time(?:stamp)?\b/.match?(sql_type)
             super || 0
           else
             super

activerecord/lib/active_record/connection_adapters/mysql/schema_creation.rb

@@ -25,6 +25,14 @@ module ActiveRecord
           end
 
           def add_column_options!(sql, options)
+            # By default, TIMESTAMP columns are NOT NULL, cannot contain NULL values,
+            # and assigning NULL assigns the current timestamp. To permit a TIMESTAMP
+            # column to contain NULL, explicitly declare it with the NULL attribute.
+            # See http://dev.mysql.com/doc/refman/5.7/en/timestamp-initialization.html
+            if /\Atimestamp\b/.match?(options[:column].sql_type) && !options[:primary_key]
+              sql << " NULL" unless options[:null] == false || options_include_default?(options)
+            end
+
             if charset = options[:charset]
               sql << " CHARACTER SET #{charset}"
             end

activerecord/lib/active_record/connection_adapters/mysql/schema_definitions.rb

@@ -75,6 +75,11 @@ module ActiveRecord
 
           super
         end
+
+        private
+          def aliased_types(name, fallback)
+            fallback
+          end
       end
 
       class Table < ActiveRecord::ConnectionAdapters::Table

activerecord/lib/active_record/connection_adapters/mysql/schema_dumper.rb

@@ -30,7 +30,10 @@ module ActiveRecord
           end
 
           def schema_type(column)
-            if column.sql_type == "tinyblob"
+            case column.sql_type
+            when /\Atimestamp\b/
+              :timestamp
+            when "tinyblob"
               :blob
             else
               super
@@ -38,7 +41,7 @@ module ActiveRecord
           end
 
           def schema_precision(column)
-            super unless /time/.match?(column.sql_type) && column.precision == 0
+            super unless /\A(?:date)?time(?:stamp)?\b/.match?(column.sql_type) && column.precision == 0
           end
 
           def schema_collation(column)

activerecord/test/cases/defaults_test.rb

@@ -100,11 +100,21 @@ if current_adapter?(:Mysql2Adapter)
     include SchemaDumpingHelper
 
     if ActiveRecord::Base.connection.version >= "5.6.0"
-      test "schema dump includes default expression" do
+      test "schema dump datetime includes default expression" do
         output = dump_table_schema("datetime_defaults")
         assert_match %r/t\.datetime\s+"modified_datetime",\s+default: -> { "CURRENT_TIMESTAMP" }/, output
       end
     end
+
+    test "schema dump timestamp includes default expression" do
+      output = dump_table_schema("timestamp_defaults")
+      assert_match %r/t\.timestamp\s+"modified_timestamp",\s+default: -> { "CURRENT_TIMESTAMP" }/, output
+    end
+
+    test "schema dump timestamp without default expression" do
+      output = dump_table_schema("timestamp_defaults")
+      assert_match %r/t\.timestamp\s+"nullable_timestamp"$/, output
+    end
   end
 
   class DefaultsTestWithoutTransactionalFixtures < ActiveRecord::TestCase

activerecord/test/cases/migration/change_schema_test.rb

@@ -269,6 +269,8 @@ module ActiveRecord
 
         if current_adapter?(:PostgreSQLAdapter)
           assert_equal "timestamp without time zone", klass.columns_hash["foo"].sql_type
+        elsif current_adapter?(:Mysql2Adapter)
+          assert_equal "timestamp", klass.columns_hash["foo"].sql_type
         else
           assert_equal klass.connection.type_to_sql("datetime"), klass.columns_hash["foo"].sql_type
         end

activerecord/test/cases/primary_keys_test.rb

@@ -291,6 +291,14 @@ class PrimaryKeyAnyTypeTest < ActiveRecord::TestCase
     schema = dump_table_schema "barcodes"
     assert_match %r{create_table "barcodes", primary_key: "code", id: :string, limit: 42}, schema
   end
+
+  if current_adapter?(:Mysql2Adapter) && subsecond_precision_supported?
+    test "schema typed primary key column" do
+      @connection.create_table(:scheduled_logs, id: :timestamp, precision: 6, force: true)
+      schema = dump_table_schema("scheduled_logs")
+      assert_match %r/create_table "scheduled_logs", id: :timestamp, precision: 6/, schema
+    end
+  end
 end
 
 class CompositePrimaryKeyTest < ActiveRecord::TestCase

activerecord/test/schema/mysql2_specific_schema.rb

@@ -6,6 +6,11 @@ ActiveRecord::Schema.define do
     end
   end
 
+  create_table :timestamp_defaults, force: true do |t|
+    t.timestamp :nullable_timestamp
+    t.timestamp :modified_timestamp, default: -> { "CURRENT_TIMESTAMP" }
+  end
+
   create_table :binary_fields, force: true do |t|
     t.binary :var_binary, limit: 255
     t.binary :var_binary_large, limit: 4095