how can i compare a record in an entity with a record in another.

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I have 2 entities studymaterial issue(Event entity) and study material stock(baseplus entity) . study material issue has many-to-one relationship with studymaterial stock. In studymaterial issue there shows studymaterial stock. In studymaterial issue when i choose a study material stock and save , there has to reduce the countof studymaterialstock. how can i campare issued study material with stock avaliable in studymaterial stock.ie;