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According to the documentation, only field names starting with `$` are reserved. But it seems that because of how the wire protocol is implemented (looking at what the drivers do, the documentation is a bit sparse here), certain field names take on a special meaning, such as "orderby" or "query":
> db.test.insert({_id:123, orderby: 1, query: 2});
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> db.test.insert({_id:124, orderby: 2, query: 1});
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> db.test.find({orderby: 1})
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{ "_id" : 123, "orderby" : 1, "query" : 2 }
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> db.test.find({query: {}, orderby: 1})
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error: { "$err" : "sort must be an object or array", "code" : 13513 }
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The wire protocol should be updated to use some kind of different scheme to avoid ambiguity, such as prefixing the special parts of the query with $ as happens in other places.
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