An example of how to add a custom predicate to Arel

custom_arel.rb

module Arel::Predications
  def has_key(right)
    Arel::Nodes::HasKey.new(self, quoted_node(right))
  end
end

class Arel::Nodes::HasKey < Arel::Nodes::Binary
  def operator; :"?" end
end

class Arel::Visitors::PostgreSQL
  private
  def visit_Arel_Nodes_HasKey o, collector
    infix_value o, collector, " #{Arel::Nodes::HasKey.new(nil, nil).operator} "
  end
end

example.rb

Person.where(Person.arel_table[:address].has_key('borough')).to_sql
# "SELECT \"people\".* FROM \"people\" WHERE (\"people\".\"address\" ? 'borough')"