Call to undefined method Builder::affectedRows()

You're right !

An other solution is :
the method update() should return FALSE if failure and the number of affected rows if success

The signature of the method would be :
public function update($withAffectedRows = FALSE)

It's better with a parameter because we don't always want the number of affected rows and it costs a query ...

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My mistake, the method affectedRows() doesn't perform a database query ...