I like keys. Fun but also useful to learn bass.
For somebody that wants to learn about chords ( and I include myself because there's always something more to learn about chords):-
Start off by learning the fingering position for C-scale on a bass (4-string shown here).:-
Once you know this fingering it will be the same virtually every key on the E-string.
If you have an electronic keyboard...many of them show you the chord you are playing with your left hand when "accomp" is on.:-
This is useful because you can see what you are playing......how it's fingered (if you play it and don't do anything it will "loop" repeatedly until you change it).
So you can go to your piano chord dictionary :-
and learn lots of chords this way by fingering position.
Now if you wanted to know what a C7 was on bass....you would pick those C7 notes out on the scale you learned in the first pic.
With the keyboard running on a chord on "loop" (in this case C7) you can now hear how your bass sounds to it and how other notes in that chord co-relate to the sound.
If you know the fingering for C7....it will sound the same when played like that for the key of A etc etc etc etc because the grid fingering pattern doesn't change...only the key does.
So it's pretty easy to co-relate the notes on the keyboard:-
.....to notes on the C-grid scale on bass.
If you have a good memory for finger positioning you can play the same chord type and root notes etc. in other keys right away.
Personally I like this learning method for any type of chord STRUCTURE and how it can be transposed to bass. More so than a lot of bass-chord books where you can't hear the representation/comparison.
So keys tend (for me) to make it more fun when advancing on bass.