How are the members in a cabinet (cabinet ministers) selected in a coalition government?

It is always the prerogative of the PM to select his colleagues in the Cabinets. On his recommendation, the President appoint the Cabinet Ministers. But, in a coalition arrangement, due to the coalition compulsion, the alliance partners come to an understanding as to who will become ministers and even the portfolios are pre-determined. It is only a matter of formality for the PM to recommend them for the Cabinet.