The Many side in a One-To-Many relationship always has the foreign key, and can be removed without affecting the referenced row on the One side. The reverse is not true: As all rows on the Many side reference one row on the One side, the foreign key on the Many side must be nullified, before the One side can be removed.
I JPA,however, please note that the relationship should be maintained on both sides, i.e.
t2.setT1(null);
t1.getT2s().remove(t2);
if t1 and t2 are POJO instances mapped to tables t1 and t2 respectively.
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