The mongo client allows specifying an index by name:

db.foo.find()._addSpecial("$hint", "index_name")

However, when issuing the "corresponding"

foo.find({}, :hint => 'index_name')

Then 'index_name' is interpreted as a field name, resulting in the javascript equivalent:

db.foo.find()._addSpecial("$hint",

)

While that might be a sensible default, there ought to be a way to specify an index hint by name.

As the current behavior is documented, changing the semantics of :hint would definitely be a breaking change, but a workaround would be to add a separate option :named_hint or something along those lines.