+ elsif expr = /^ *\( *(\w+) *(<=?|>=?|=) *(\w+) *\) *$/.match(attr)
+ if operator != '=' || ![true,"true"].index(operand)
+ raise ArgumentError.new("Invalid expression filter '#{attr}': subsequent elements must be [\"=\", true]")
+ end
+ operator = expr[2]
+ attr1, attr2 = expr[1], expr[3]
+ allowed = attr_model_class.searchable_columns(operator)
+ [attr1, attr2].each do |tok|
+ if !allowed.index(tok)
+ raise ArgumentError.new("Invalid attribute in expression: '#{tok}'")
+ end
+ col = attr_model_class.columns.select { |c| c.name == tok }.first
+ if col.type != :integer
+ raise ArgumentError.new("Non-numeric attribute in expression: '#{tok}'")
+ end
+ end
+ cond_out << "#{attr1} #{operator} #{attr2}"