Question

I am trying to create a test database that is a replica of a preexisting database. I am using Django models (theoretically the models used with the original database) in order to do this. I recently inherited the code from someone else and am trying to figure out what exactly is going on with the code.

In the model, one the of the tables has two columns identified as the primary key.

        column1 = models.IntegerField(primary_key = True)
        column2 = models.IntegerField(primary_key = True)
        column3 = models.CharField(max_length = 30)

When I try to sync this model to the test database – an error occurs :

File “/somePathHere/MySQLdb/connections.py”, line 36, in defaulterrorhandler raise errorclass, errorvalue
_mysql_exceptions.OperationalError: (1068, ‘Multiple primary key defined’)

It is written in the django docs that Django does not allow multiple primary keys. However, looking at the output of DESCRIBE [tablename] in the original MySQL database, it seems like that is exactly what is going on here :

        +------------+------------+------+-----+---------+-------+
        | Field      | Type       | Null | Key | Default | Extra |
        +------------+------------+------+-----+---------+-------+
        | IDENTIFIER | bigint(20) | NO   | PRI | NULL    |       |
        | TIMESTAMP_ | bigint(20) | NO   | PRI | NULL    |       |
        | VALUE_     | longtext   | YES  |     | NULL    |       |
        +------------+------------+------+-----+---------+-------+

Note that both IDENTIFIER and TIMESTAMP_ are listed as the primary keys.

I’m seeing a lot of topics on SO ( Example 1 , Example 2, and Example 3 ) about creating a primary key based on multiple columns – is what I am seeing a composite key? In which case, how is this relayed via a Django model, i.e. how would one replicate it?

If it is not a composite key, what is it?

Answer 1

It’s a composite primary key. Try executing this:

show create table mytable;

It should show you the definition of the composite key.

This is nothing “unusual” from a mysql perspective.