There are 2(two) Node types

  1. A
  2. B

A has a node reference field to node type B.

If a node x of A type used a node y of B type as a node-reference then next time node z of A type will not use the node y of B type as a node-reference because x is already used as node-reference.

  • Did you already try the Entityreference unique or Field validation module? – Watergate Aug 4 '15 at 10:45
  • No, but I think Entityreference unique will be perfect though I've not found the recommended version. – johirpro Aug 4 '15 at 11:00
  • @johirpro what do you mean by recommended version? It does not have a stable release yet, but rc means release candidate - maintainer thinks it's ready and all he needs is some brave people who will start using it and confirm it's ready :) By using it and reporting everything you didn't found to work as expected, you help maintainers to help you. – Mołot Aug 4 '15 at 11:08
  • Thanks! @Mołot to explain about the meaning of rc. I'll use the stable release. – johirpro Aug 4 '15 at 11:13
  • The Git repository is indeed a bit messy, there is also an issue (more than a year old) of applying Drupal's coding standards.. I would use the rc version. – Watergate Aug 4 '15 at 11:16

The Entityreference unique sounds what you're looking:

This modules adds a checkbox to an entity reference instance to enable a check for unique multiple entity reference fields. With this option enabled it is not possible to add one and the same entity to a multiple entity reference field.

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