Dropdown list filtering

[Jacek88]

Hi i have two list:

1 Project list (that takes requests from request list)

2 Request list

I want to make a dropdown list in project but i want to see only untaken requests.

Example:

Project 1:

dropdown list:

Request a (selected)

Request b (selected)

Request c

after a while i make Project 2 where only request c is avilable:

Project 2:

dropdown list:

Request c

(a and b were taken i project 1)

how to do it?

[#2]

You have to provide some custom UI to achieve this since SharePoint List Forms out of the box don't allow you to specify such behavior. 

There are several ways you can consider to provide this custom UI:

• Use PowerApps to customize your list forms (I'm not 100% sure you can achieve what you want). 
• Use SharePoint Framework extensions (Field Customizers)
  https://docs.microsoft.com/en-us/sharepoint/dev/spfx/extensions/overview-extensions
• Manage your list content using a custom SharePoint Framework WebPart (and provide some way to prevent managing the lists using the default UI?)
• Provide a custom (web) application making use SharePoint API's manage your list

Hope this helps!

[#3]

@Paul Pascha I tried to do a "invisible" list (request not taken) that moves items to "finished request list" after the project has been created ... my problem was that I can't manage this in Microsoft Flow because I lost link connection to items in finished items...