Looks like you're trying to find the schedule using the project's id.
I'd do this in your controller:
# find the parent model first
@project = Project.find params[:project_id]
# now get its associated schedule:
@schedule = @project.schedules.find params[:id]
You also might have the ids backwards in your link_to helper. You can try using the shortcut:
link_to 'Schedule', [project, schedule]
That gets converted to the project_schedule_path