How to be able to use a content once in a content field? I created a field in to a content. Let's call the content type National Anthem and the field Country. Country is a content type too.

What I'd like to do is to forbid the use of a country which has been chosen already.
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For example, let's say that we have the country Germany, France and United-Kingdom. If I create a new National Anthem and for the field Country, I choose United-Kingdom. I now create a new National Anthem and I can only choose between France and Germany.

Thank you.
Assuming you are using an entity reference (default reference in Drupal 8, and provided by the Entity Reference module for Drupal 7). You can do this with an entity reference view: 



Create a new view that lists content of type Country. Then take the following steps:


Make sure to add a new Entity Reference Display.
Enable the title field as Search field in the formatter settings. (Behind drop down menu.)
Add a relation of type Content using field_country.
Filter content that has no value for ID using the relationship. 
(optional), if you reference country from more content types than National Anthem, filter also for the Content type to be of National Anthem; again using the relationship.


This view will select all countries, which are used on nodes with no ID. (Thus are not referenced.)

Finally, make sure you enable the view in the content type's field settings: