how to solo on bass in a given key

the scale is a good start and sort of tells you what notes you should and which you shouldn't play in a solo. but just playing a scale over a chord progression is hardly a good solo.

you probably want to learn how to improvise bass solos? the easiest way is to learn as much solos as possible transpose them to different keys, take bits and pieces of other solos, add something of your own that you really like and before you know it you're improvising a cool bass solo.

another way is to actually try to sing or hum what you want your bass solo to be and then try to play it on the bass. do that long enough and you'll gain the ability to do this straight on the bass, without the intermediate hum.