Add a extra field to ActiveModel Serializer

Lets say I am having a Model called Vendor(id, name, lat, lon, created_at). And i have a query in which i find distance of vendors from the current latitude and longitude.

Query is -

query = "*, ST_Distance(ST_SetSRID(ST_Point(#{lat}, #{lon}), 4326)::geography,ST_SetSRID(ST_Point(lat, lon), 4326)::geography) as distance"

Vendor.select(query)

I have serializer class as -

class VendorSerializer < ActiveModel::Serializer
    attributes :id,
               :lat,
               :lon,
               :name,
               :created_at

    def attributes
        hash = super
        # I tried to override attributes and added distance but it doesn't add
        hash[:distance] = object.distance if object.distance.present?
        hash
    end
end

I want to have a serialised object as {id, lat, lon, name, created_at, distance}. So the model attributes get attached but how can i add the extra field/attribute i.e, "distance" to the serialised hash?