Migrate failure: on clause is ambiguous

Question

I'm working with the migrate module for the first time and I've worked with the db_select api a little bit, but I'm running into an error.

I'm trying to convert some old labels into a drupal taxonomy. When I browse to the /admin/content/migrate page, it does my query correctly and shows the right number of rows that should be migrated. However, when I actually try to run the import, I get this error:

Migration failed with source plugin exception: SQLSTATE[23000]: Integrity constraint violation: 1052 Column 'labelId' in on clause is ambiguous

Here is the code I have in my LabelMigration class:

$this->map = new MigrateSQLMap($this->machineName,

    array(
        'labelId' => array('type' => 'int', 'not null' => TRUE)
    ),
    MigrateDestinationTerm::getKeySchema()
);
$query = db_select('migrate_account_label', 'l')
         ->fields('l', array('labelId', 'label'))
;

$query->leftJoin('migrate_asset_labels_membership', 'lm', 'l.labelId = lm.labelId');
$query->leftJoin('migrate_asset', 'a', 'lm.assetId = a.assetId');
$query->addExpression('count(*)', 'num');
$query->condition('a.type', 'B');
$query->groupBy('l.labelId');
$query->groupBy('l.label');

$this->source = new MigrateSourceSQL($query);

// Set up our destination - terms in the vocabulary
$this->destination = new MigrateDestinationTerm('article_labels');

$this->addFieldMapping('name', 'label');
$this->addFieldMapping('description')
    ->defaultValue('');

If I remove the leftJoin() calls then obviously the error about the on statement goes away, but I don't understand how labelId is ambiguous because I'm always referencing it with a table alias.

Any ideas where this error is coming from? Is it coming from the MigrateSQLMap at the top? If so, how can I reference that labelId with an alias to make it non-ambiguous? I tried making it l.labelId, but that didn't work.

Answer 1

Figured it out!

When defining the field in the MigrateSQLMap I can set a table alias for the field:

$this->map = new MigrateSQLMap($this->machineName,
    array(
        'labelId' => array(
            'type' => 'int',
            'not null' => TRUE,
            'alias' => 'l', // it's the letter small case "L", not the digit 1
        )
    ),
    MigrateDestinationTerm::getKeySchema()
);

The 'alias' => 'l' makes the labelId become l.labelId in the queries.

Answer 2

I thought that he was setting alias to 1 (ie, boolean true), but there's a reason it's a string - that is the actual alias, so if you're alias is n, 'alias' => 'n'. That actually cost me about 1/2 hr.