Accessing QID's from a DMatrix

I’d like to be able to retrieve the qid’s from a dmatrix in a custom objective function, however, there’s no direct access, and there’s no getter, i’ve tried:

dm = xgb.DMatrix([[1, 2], [1, 2], [3, 4]], [1, 1, 0], qid=[1, 1, 2])

but that gives: Unknown uint32 field name: qid

what am i missing?

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answering my own question: i realized that


still works, and this can be used to reproduce a set of query ids like:

    groups = dm.get_group()
    qids = reduce(lambda acc, x : acc + [x[0]]*x[1], enumerate(groups), [])
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