This method regenerates the left/right ids for the nested set based on the
parent/child relations. This function executes three queries per item, so
it should only be called, when the set has one of the following problems:
- The set has a duplicated value inside the left/right id chain
- The set has a missing value inside the left/right id chain
- The set has items that do not have a left/right is set
When regenerating the items, the items are sorted by parent id and their
current left id, so the current child/parent relationships are kept and
running the function on a working set will not change any orders.
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