Hi Khanh,
If I understand you well, you should use drop-down list form control on your form and chose 'Macro expression as datasource. Macro expressionshould be something like this:
Documents.Where("ClassName='DancingGoat.Cafe'")
Also, it is important that in fields bellow (for 'Item transformation' and 'Value column') use proper values, like this {% DocumentName %} or {% DocumentID %}. If you need more advanced thing, you can create custom macro method and add it here.
Best regards, Dragoljub