On the Mongoid 6+ and MongoDB 3.2+ its possible to do the following index:

    index({ _slugs: 1 }, { unique: true, partial_filter_expression: { _slugs: { '$type' => 'string' } } })

This has an advantage over sparse: true because it won't consider empty array [ ] as a duplicate value (i.e. if two or more docs have _slugs: [ ]), and we could remove some unset related hacks we have.