Rails 5.0.1 db: migrate - ActiveRecord :: RecordNotUnique: Mysql2 :: Error: Duplicate record 'for key' PRIMARY ': INSERT INTO `ar_internal_metadata`

I recently upgraded to Rails 5.0.1 from 4.2.x and while trying to migrate db: I ran into an issue with ar_internal_metadata which seems to be incompatible with MySQL5.

Reproducing (assuming Rails 5 environment with gems and database schema installed):

• Drop the ar_internal_metadata table from the development database (if any).
• Run "bundle exec rake db: migrate"
• Start it up again.

On the second run (regardless of whether migrations are pending or not), this error occurs:

ActiveRecord :: RecordNotUnique: Mysql2 :: Error: Duplicate record '' for key 'PRIMARY': INSERT INTO ar_internal_metadata

( created_at

, updated_at

) VALUES ('2017-04-30 20:49:17', '2017-04-30 20:49: 17 ')

The ar_internal_metadata schema tells me that MySQL5 does have the correct concatenation: the primary key field is configured as non-empty, but it defaults to null. And, since it's a varchar (255) type, it can't auto-increment. Apparently the ActiveRecord hardware does not provide a value for the "key".

mysql> desc ar_internal_metadata;
+------------+--------------+------+-----+---------+-------+
| Field      | Type         | Null | Key | Default | Extra |
+------------+--------------+------+-----+---------+-------+
| key        | varchar(255) | NO   | PRI | NULL    |       |
| value      | varchar(255) | YES  |     | NULL    |       |
| created_at | datetime     | NO   |     | NULL    |       |
| updated_at | datetime     | NO   |     | NULL    |       |
+------------+--------------+------+-----+---------+-------+
4 rows in set (0.02 sec)

      

        
        
        
      

    

Here is the content of ar_internal_metadata just before the second call to db: migrate:

mysql> select * from ar_internal_metadata;
+-----+-------+---------------------+---------------------+
| key | value | created_at          | updated_at          |
+-----+-------+---------------------+---------------------+
|     | NULL  | 2017-04-30 20:56:39 | 2017-04-30 20:56:39 |
+-----+-------+---------------------+---------------------+
1 row in set (0.00 sec)

      

        
        
        
      

    

Has anyone seen this issue? Any suggestions for workarounds or where to look? What value should ActiveRecord put in ar_internal_metadata for "key" (especially the second time db: migrate run)?