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One of the issues I encountered when migrating nodes to Drupal, using the migrate module, was that I couldn't associate nodes with more than one taxonomy term. Actually in this example, I'm migrating content from one Drupal database to another, so I'm going to assume everyone is already familiar with the database structure, specifically the node and term_node tables.

When I first started using the migrate module, I ran into a similar problem with migrating a user's roles. It's not possible to just create a Views relationship (aka LEFT JOIN) between the node and term_node tables using the node id. This will produce one row for each node and taxonomy combination, but the migrate module is only able to handle data sets that contain one row for each entity. With the above solution, I have more than one row for each node, which causes the migrate module to import the same node more than once, causing all sorts of problems.

Like with the user roles example before, we can overcome this by implementing a migrate hook, specifically hook_migrate_prepare_node().

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