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Transformer.objects.filter(mark__iregex='^([^-]+)')\
.annotate(mark_prefix=Value(Magic('mark')))\
.values_list('mark_prefix', flat=True)
query = Transformer.objects.annotate(
mark_prefix_pos=StrIndex(F('mark'), Value('-'))
).annotate(
mark_prefix=Substr(F('mark'), 1, F('mark_prefix_pos') - 1, output_field=CharField())
)
print(query.values_list('mark_prefix', flat=True))
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