Fix schema dumper behavior about datetime precision value

By rails#42297, the default value of datetime precision in rails is now 6.
With this change, if the value of datetime precision is 0, it should be
dumped into schema, but it is not.
So I modified it to dump to schema when the value of datetime precision
is 0, and not to dump when the value is 6, which is the default value of
rails.

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

@@ -53,7 +53,9 @@ def schema_limit(column)
           end
 
           def schema_precision(column)
-            super unless /\A(?:date)?time(?:stamp)?\b/.match?(column.sql_type) && column.precision == 0
+            return if /\Adatetime\b/.match(column.sql_type) && column.precision == 6
+            return if /\Atime(?:stamp)?\b/.match?(column.sql_type) && column.precision == 0
+            super
           end
 
           def schema_collation(column)

activerecord/test/cases/date_time_precision_test.rb

@@ -193,13 +193,15 @@ def test_writing_a_blank_attribute_timestamptz
       end
     end
 
-    def test_schema_dump_includes_datetime_precision
+    def test_schema_dump_includes_non_default_datetime_precision
       @connection.create_table(:foos, force: true) do |t|
+        t.datetime :datetime_zero, precision: 0
         t.timestamps precision: 6
       end
       output = dump_table_schema("foos")
-      assert_match %r{t\.datetime\s+"created_at",\s+precision: 6,\s+null: false$}, output
-      assert_match %r{t\.datetime\s+"updated_at",\s+precision: 6,\s+null: false$}, output
+      assert_match %r{t\.datetime\s+"datetime_zero",\s+precision: 0$}, output
+      assert_match %r{t\.datetime\s+"created_at",\s+null: false$}, output
+      assert_match %r{t\.datetime\s+"updated_at",\s+null: false$}, output
     end
 
     if current_adapter?(:PostgreSQLAdapter, :SQLServerAdapter)

activerecord/test/cases/defaults_test.rb

@@ -143,7 +143,7 @@ class MysqlDefaultExpressionTest < ActiveRecord::TestCase
     if supports_datetime_with_precision?
       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(?:\(\))?" }/i, output
+        assert_match %r/t\.datetime\s+"modified_datetime",\s+precision: 0,\s+default: -> { "CURRENT_TIMESTAMP(?:\(\))?" }/i, output
       end
 
       test "schema dump datetime includes precise default expression" do

activerecord/test/cases/primary_keys_test.rb

@@ -321,7 +321,7 @@ def test_any_type_primary_key
     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: { type: :timestamp, precision: 6.* }/, schema
+      assert_match %r/create_table "scheduled_logs", id: { type: :timestamp.* }/, schema
     end
   end
 end

activerecord/test/cases/schema_dumper_test.rb

@@ -840,7 +840,7 @@ def test_schema_dump_defaults_with_universally_supported_types
     assert_match %r{t\.date\s+"date_with_default",\s+default: "2014-06-05"}, output
 
     if supports_datetime_with_precision?
-      assert_match %r{t\.datetime\s+"datetime_with_default",\s+precision: 6,\s+default: "2014-06-05 07:17:04"}, output
+      assert_match %r{t\.datetime\s+"datetime_with_default",\s+default: "2014-06-05 07:17:04"}, output
     else
       assert_match %r{t\.datetime\s+"datetime_with_default",\s+default: "2014-06-05 07:17:04"}, output
     end