Fix default value for mysql time types with specified precision

The TIME, DATETIME, and TIMESTAMP types [have supported](https://mariadb.com/kb/en/library/microseconds-in-mariadb/)
a fractional seconds precision from 0 to 6.

Default values from time columns with specified precision is read
as `current_timestamp(n)` from information schema.

rake `db:schema:dump` produces `schema.rb` **without** default values for time columns with the specified precision:

    t.datetime "last_message_at", precision: 6, null: false

rake `db:schema:dump` produces `schema.rb` **with** default values for time columns with the specified precision:

    t.datetime "last_message_at", precision: 6, default: -> { "current_timestamp(6)" }, null: false

1 files changed, 2 insertions, 2 deletions

diff --git a/activerecord/lib/active_record/connection_adapters/mysql/schema_statements.rb b/activerecord/lib/active_record/connection_adapters/mysql/schema_statements.rb
index ce50590651..2087938d7c 100644
--- a/activerecord/lib/active_record/connection_adapters/mysql/schema_statements.rb
+++ b/activerecord/lib/active_record/connection_adapters/mysql/schema_statements.rb
@@ -80,8 +80,8 @@ module ActiveRecord
 
           def new_column_from_field(table_name, field)
             type_metadata = fetch_type_metadata(field[:Type], field[:Extra])
-            if type_metadata.type == :datetime && /\ACURRENT_TIMESTAMP(?:\(\))?\z/i.match?(field[:Default])
-              default, default_function = nil, "CURRENT_TIMESTAMP"
+            if type_metadata.type == :datetime && /\ACURRENT_TIMESTAMP(?:\([0-6]?\))?\z/i.match?(field[:Default])
+              default, default_function = nil, field[:Default]
             else
               default, default_function = field[:Default], nil
             end