Using actsasnestedset in ruby and something like taxon.movetochildof(parent) you can get this error if your table data is incomplete. You may have functioning nesting through the chain of parent IDs but you need the left and right fields as well. If either the left or right field is null you will get an error like this one for certain move operations. This can easily happen if you’ve created some dodgy fixtures for instance.

Trying using Taxon.rebuild! to repair the table.

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