alembic/env.py target_metadata = metadata "No module name al_test.models"

When I use alembic to control the version of my project's database,part of codes in env.py like:

# add your model's MetaData object here
# for 'autogenerate' support
# from myapp import mymodel
# target_metadata = mymodel.Base.metadata
from al_test.models import metadata

target_metadata = metadata

when I run 'alembic revision --autogenerate -m "Added user table"', I get an error : File "alembic/env.py", line 18, in from al_test.models import metadata ImportError: No module named al_test.models

so how to solve the question? thanks!