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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _get_flat_db_sources(self, model):
 """ Return a flattened representation of the individual ``sources`` lists. """

        sources = []
        for source in self.sources:
            for sub_source in self.expand_source(source):
                target_field = self.resolve_source(model, sub_source)
                if target_field:
                    sources.append(sub_source)
        return sources