Light box for ceiling light is attached to ceiling beam
Several manufacturers make that box.  It's specifically for mounting ceiling fans or other heavy fixtures, which can then be screwed directly into the wood of the joist.

Here is one such box.  Or here. 

It's perfectly ok to use for a standard fixture, not just ceiling fans.  It's just designed specifically to be able to screw heavy mounting screws directly into the wood of the joist.