Basically, you can't. M2m fields are saved after the model is saved, so you
either get objects that are not yet ready for that check (as you
experienced) or you will test the previous value of the m2m field, neither
of which is what you want.
You can modify the admin interface and do the validation in the AdminForm,
or perhaps catch the m2m_changed signal and do some checks there, but you
can't do the validation you want in the clean method.
mjl
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