In MongoDB this is not possible:

> db.test.insert({"foo.bar": 1}); Mon Jul 26 14:24:39 JS Error: uncaught exception: can't have . in field names [foo.bar]

After all that's only natural, because the dot notation is supported in queries.

For example, consider storing an e-mail in MongoDB:

{ content: "<p>HTML mail. <img src="cid:1234.0" alt="from attachment"/></p>", attachments: { "1234.0": { some details about the attachment } } }

This is not legal, but in this case it's of course easy to work around (have attachments in an array and have the CID as a value instead of a key).