Add ability to use environment variables with no database name in the url

[nduthoit]

Allows for the use of database URL environment variables with no database name specified. This will in turn result in less environment variables needed to be set when there are many shards per database node.

For example imagine an app with 1 primary database and 8 shards distributed over 3 database servers:

DB_NODE_001_URL=postgres://user:passwd@node001.company.org:5432
DB_SHARD_NODE_001_URL=postgres://user:passwd@shardnode001.company.org:5432
DB_SHARD_NODE_002_URL=postgres://user:passwd@shardnode002.company.org:5432

    {
        'unsharded_databases': [
            {
                'name': 'DB01',
                'environment_variable': 'DB_NODE_001_URL',
                'default_database_url': 'postgres:://...',
                'database_name': 'db01'
            }
        ],
        'sharded_databases': [
            {
                'name': 'SHARD_01',
                'environment_variable': 'DB_SHARD_NODE_001_URL',
                'default_database_url': 'postgres:://...',
                'database_name': 'shard01',
                'shard_group': 'default',
            }, {
                'name': 'SHARD_02',
                'environment_variable': 'DB_SHARD_NODE_001_URL',
                'default_database_url': 'postgres:://...',
                'database_name': 'shard02',
                'shard_group': 'default',
            }, {
                'name': 'SHARD_03',
                'environment_variable': 'DB_SHARD_NODE_001_URL',
                'default_database_url': 'postgres:://...',
                'database_name': 'shard03',
                'shard_group': 'default',
            }, {
                'name': 'SHARD_04',
                'environment_variable': 'DB_SHARD_NODE_001_URL',
                'default_database_url': 'postgres:://...',
                'database_name': 'shard04',
                'shard_group': 'default',
            }, {
                'name': 'SHARD_05',
                'environment_variable': 'DB_SHARD_NODE_002_URL',
                'default_database_url': 'postgres:://...',
                'database_name': 'shard05,
                'shard_group': 'default',
            }, {
                'name': 'SHARD_06',
                'environment_variable': 'DB_SHARD_NODE_002_URL',
                'default_database_url': 'postgres:://...',
                'database_name': 'shard06',
                'shard_group': 'default',
            }, {
                'name': 'SHARD_07',
                'environment_variable': 'DB_SHARD_NODE_002_URL',
                'default_database_url': 'postgres:://...',
                'database_name': 'shard07',
                'shard_group': 'default',
            }, {
                'name': 'SHARD_08',
                'environment_variable': 'DB_SHARD_NODE_002_URL',
                'default_database_url': 'postgres:://...',
                'database_name': 'shard08',
                'shard_group': 'default',
            },
        ]
    }

[nduthoit]

We could also override the existing value of db_config['NAME'] even if it is set by replacing this with:

if database_name is not None

[JBKahn]

Hmmm I agree that if there's a precedence it should be the name you set, rather than the one in the URL.

[coveralls]

Coverage decreased (-0.1%) to 89.717% when pulling f32d5c3 on nduthoit:optional-database-name-in-url into eb6d9ab on JBKahn:master.

[JBKahn]

add a test to https://github.com/JBKahn/django-sharding/blob/master/tests/test_settings_helpers.py

[JBKahn]

What happens if you don't have a name in the args or the URL?

[coveralls]

Coverage increased (+0.03%) to 89.846% when pulling c95406a on nduthoit:optional-database-name-in-url into eb6d9ab on JBKahn:master.

[coveralls]

Coverage increased (+0.03%) to 89.846% when pulling c95406a on nduthoit:optional-database-name-in-url into eb6d9ab on JBKahn:master.

[coveralls]

Coverage increased (+0.03%) to 89.846% when pulling c95406a on nduthoit:optional-database-name-in-url into eb6d9ab on JBKahn:master.

[nduthoit]

@JBKahn same as what would happen now if it's not in the URL: it wouldn't be set

I didn't want to change the existing behaviour which doesn't validate the URL (other than using dj-database-url)

[JBKahn]

I'll release it this weekend and update the changelog and all that.