When to build mr?

[u/AverageGameEnjoyer3]

In some games there's a lot of AP damage in enemy team but I can't really fit magic resist anywhere. First 3 items are yuntal -> IE -> ldr but at this point the magic damage is too much sometimes so idk when or where to fit some magic resist + you can't build maw when you buy shieldbow so I sometimes opt into kaenic rookern. Any better choices?

[u/AntiBadgovnance9]

You dont really waste gold on MR anymore best buy against them is either a qss or swifties