Expected Behaviour of Compound _id in MongoDB?

The index is not a compound index, but an index on the whole value of the id field. MongoDB does not look into an indexed field, and instead uses the raw BSON representation of a field to make comparisons (if I read the docs correctly).

You would need a separate compound index for the behavior you desire. In general I recommend against using objects as _id because key order is significant in comparisons, so {a:1, b:1} does not equal {b:1, a:1}. Since not all drivers preserve key order in objects it is very easy to shoot yourself in the foot by doing something like this.

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