Signs/symptoms vs definitive diagnosis coding

If a patient with migraine headache has a severe bout of nausea and vomiting causing dehydration that requires an IV and a Phenergan injection, would the diagnosis code for migraine headache (346.2X) still be used for the diagnosis or would dehydration plus nausea and vomiting be the appropriate diagnoses? At what point would migraine headache no longer be the primary diagnosis?

Thank you.

Why not use all the codes?

Who says you have to use only one? For the IV fluids, I would put dehydration as #1 but phenergan can be linked to either migraine or nausea with migraine.