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)
```

4 files changed, 26 insertions, 1 deletions

diff --git a/activerecord/test/cases/defaults_test.rb b/activerecord/test/cases/defaults_test.rb
index 6532efcf22..a6297673c9 100644
--- a/activerecord/test/cases/defaults_test.rb
+++ b/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
diff --git a/activerecord/test/cases/migration/change_schema_test.rb b/activerecord/test/cases/migration/change_schema_test.rb
index 48cfe89882..1d305fa11f 100644
--- a/activerecord/test/cases/migration/change_schema_test.rb
+++ b/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
diff --git a/activerecord/test/cases/primary_keys_test.rb b/activerecord/test/cases/primary_keys_test.rb
index 03c8644229..12386635f6 100644
--- a/activerecord/test/cases/primary_keys_test.rb
+++ b/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
diff --git a/activerecord/test/schema/mysql2_specific_schema.rb b/activerecord/test/schema/mysql2_specific_schema.rb
index 9a203a7293..90a314c83c 100644
--- a/activerecord/test/schema/mysql2_specific_schema.rb
+++ b/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