How to use sed command to add a string before a pattern string?

I want to use sed to modify my file named "baz".

When i search a pattern foo , foo is not at the beginning or end of line, i want to append bar before foo, how can i do it using sed?

Input file named baz:
blah_foo_blahblahblah
blah_foo_blahblahblah
blah_foo_blahblahblah
blah_foo_blahblahblah

Output file 
blah_barfoo_blahblahblah
blah_barfoo_blahblahblah
blah_barfoo_blahblahblah
blah_barfoo_blahblahblah