Array of fields to update to an expression in case of a duplicate record.
This variable is a nested array in the following format:
<some field> => array( 'condition' => <condition to execute, as a string>, 'arguments' => <array of arguments for condition, or NULL for none>, );
Type: array
protected $expressionFields = array();
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https://api.drupal.org/api/drupal/core!lib!Drupal!Core!Database!Query!Update.php/property/Update::expressionFields/8.1.x