PG Expression Indexes Produce Undefined Method `join` for String

[bousquet]

I have a case-insensitive index on User#username which produces the following index definition:

#<struct ActiveRecord::ConnectionAdapters::IndexDefinition table="users", name="index_users_on_lowercase_username", unique=true, columns="lower((username)::text)", lengths=[], orders=nil, where=nil, type=nil, using=:btree, comment=nil>

Because the #columns attribute of that struct is a String, we get the following errors when annotating our model:

Unable to annotate app/models/user.rb: undefined method `join' for "lower((username)::text)":String
Did you mean?  JSON
Unable to annotate app/models/user.rb: no implicit conversion of nil into Array
Model files unchanged.