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The central class in the Migrate module - the one you'll spend most of your time dealing with - is the
Migration class. You derive your own classes from
Migration, each of which represents the migration of data from a given source (such as a query against a source database) to a given Drupal destination (such as nodes of a specific content type, like 'article').
You embed in this class instances of four other classes:
MigrateSourcerepresents the source data. The Migration class iterates over row objects generated by this class.
MigrateDestinationrepresents the destination object (user, node, etc.). Each row produced by MigrateSource is manipulated and passed on to the destination class to create the object.
MigrateMaprepresents the relationship between source and destination objects, keeping track of precisely which source data record resulted in the creation of which Drupal object.
MigrateFieldMappingrepresents a mapping between a source field and a destination field. A typical Migration subclass will contain several field mappings.