Postgres-Queries in Rails: where not
===

I use Rails with PostgreSQL and it works perfectly for queries like this:

    # Does hstore_column contain key?

    scoped_users = User.where('hstore_column ? :key', key: 'name_of_key')

Now my problem: How can I invert this query / how can I find all users which **do not** have a specific key specified?

    # I tried this query amongst others, but it does not work:

    scoped_users = User.where('hstore_column ? :key = FALSE', key: 'name_of_key')

Thanks for you help!