mydb = db.getSisterDB( "config" );

t = mydb.foo;
t.drop();

t.insert( { x : 1 } );
res1 = mydb.runCommand( "dbhash" );
assert( res1.fromCache.indexOf( "config.foo" ) == -1 );

res2 = mydb.runCommand( "dbhash" );
assert( res2.fromCache.indexOf( "config.foo" ) >= 0 );
assert.eq( res1.collections.foo, res2.collections.foo );

t.insert( { x : 2 } );
res3 = mydb.runCommand( "dbhash" );
assert( res3.fromCache.indexOf( "config.foo" ) < 0 );
assert.neq( res1.collections.foo, res3.collections.foo );