unique identifier to distinguish an instance

so I have two models, A and B. B has a foreignkey relationship in it
from A (To be clear because I know I dont explain it that well, one A
has many Bs). for each group of Bs each A is connected with, I want
there to be a way to mark one of the Bs as unique from the rest of
them. I added in an extra field called unique_identifier that is a
booleanfield. The problem is I dont know the best way to go about
making sure there is only one B instance marked True for the
unique_identifier field for each set of Bs. What would be a good way
to ensure there is one and only one instance marked True?

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