`validates_uniqueness_of` would blow up when used with a scope that was
an array column. The validator must perform a query to determine whether
the record already exists in the database, and uses the scope to do so.
When it constructs the query, it deserializes the value of the scope (an
array). Unfortunately this breaks the query because Postgres is still
expecting that value to be an array. There is a newer version of Arel
that prevents this from happening, but 4.1 is not on that version, so we
have to emulate that code here.