Rails Active Admin - how to set up a form's with a drop down that includes X choices plus a free input
There's nothing in ActiveAdmin (Formtastic) that helps you with that, but a popular option often used with ActiveAdmin is Select2, which might help, see for example Select 2 version 4.0 allow user to enter free text
Create a drop down menu using a Table Attribute in Rails Active Admin
I think you are looking for something as follows:
f.input :membership_code, as: :select, collection: Member.all.map(&:membership_code)
Dynamically-adjusting drop down fields on Active Admin form (rails 3.2)
You'll need to do either a page refresh to update data based on what's selected or (my preference) an AJAX call to update the other dropdown after the first one is selected.
There's a great railscast on doing just this - that would be a great place to start.
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