I found this problem before, but I'm not sure how to modify it. The reason is:

Lua table index is begin from 1, but array in mongodb is 0. So there will be a
undefined element in mongodb array whose index is 0.

If we modify the driver to make index=index-1 when insert into mongodb and
index=index+1 when read from mongodb, the "undefined" will be eliminated.
We will found that the tab[1]=aa inserted into mongodb will be tab[0]=aa actual
stored in mongodb, and it's conflict with other language drivers.

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Best Regard.