You seem to imply that there is only a single "table" or "collection in the MongoDB speak, correct?
In that case the id as you've defined it must contains the table name, and each of the properties making up the id. But one has to admit that it does not make a really readable id value. anyone has alternative serialization approaches?

I do agree with you that a non flat copy of the real id makes sense for query purposes.
I'm tempted to rename 'tableName' with '_table' (or '_tableName' if you insist). It will make clear that we are talking about a technical field.

Thoughts?