When and How to load/fill out data on a Form (CRUD)

I am making some CRUD Forms, but I can’t figure it out WHEN load the comboBoxs with its respective data (like Province/State, Municipality, Town, and a lot more data that must be retrieve from database).
I don’t want to load all that information when the Form is instantiated, just when Create or Edit event are trigged otherwise, it must remains empty.
How do u handle this?

You will need a lazy Container as data source for your ComboBox:

Have a look at the add-on viritin (

It has a LazyComboBox component to makes things easy for you: