1. A computer-implemented method to merge topics associated with a content item of an online application, the method comprising: receiving a request to merge a first topic with a second topic, the first and second topics stored as separate records in a database table, each record having a first field to store a topic identifier that identifies its respective topic and a second field to store a topic identifier into which the respective topic has been merged; and responsive to reading the second field of the record for the second topic to determine that the second topic has not been merged with another topic, using processor-implemented topic merge logic to write the topic identifier of the second topic into the second field of the record for the first topic to indicate that the first topic has been merged into the second topic.