Find multiple strings in the same file using grep. Pass the results of grep to xargs with grep.
Find files containing both the add() method and the create() method.
$ grep -r -l "def add(" . | xargs grep -l "def create(" ./db/models/fields/related_descriptors.py ./contrib/contenttypes/fields.p
Edit the resulting files by enclosing the grep command in $().
$ vim $(grep -r -l "def add(" . | xargs grep -l "def create(")