How to create 3 dropdowns menus, each based on the previous one?

I have 4 entities, client, project, work order and activity. I would like to create 3 drop-down menus where in the first I choose from all the customers in the database, in the second all the projects associated with that client and in the third all the work orders associated with that project. Once selected all 3 I would like to create (through a button for example) a new activity based on that order of work. How can I do that? 
1 answers

You are looking for the constrained by function. See the documentation here: This way you can set constraines on the second and third dropdown.