Problem loading AdaptiveMetropolis sampler state from db

pymc-problem-am-db.py

import os
import pymc
database_file = "mc.pickle"
number_samples = 1000
a = pymc.Normal("a",mu=0, tau=1)
b = pymc.Normal("b",mu=0, tau=1)
mu = a+b
c = pymc.Normal("c", mu=mu, tau=1, observed=True, value=0.5)


if os.path.exists(database_file):  #restore existing chain and do another number_samples samples
	db = pymc.database.pickle.load(database_file)
	mcmc = pymc.MCMC([a,b,c], verbose=2, db=db)
	#  I cannot find any combination of the following lines that works!
	# mcmc.assign_step_methods()
	# mcmc.use_step_method(pymc.AdaptiveMetropolis, [a,b], delay=100, interval=100)
	# mcmc.restore_sm_state()
	# mcmc.restore_sampler_state()
else: #Start new chain
	mcmc = pymc.MCMC([a,b,c], verbose=2, db='pickle', dbname=database_file)
	mcmc.use_step_method(pymc.AdaptiveMetropolis, [a,b], delay=100, interval=100)

print "Initial covmat:"	
print mcmc.step_method_dict[a][0].C
mcmc.sample(number_samples)
print "Final covmat:"
print mcmc.step_method_dict[a][0].C

mcmc.db.commit()
mcmc.db.close()