How do I build a JSON object?

Currently I build a JSON object by doing:

@users = User.all

@users.each do |user|
  @userlist << {
    :id => user.id,
    :fname => user.fname,
    :lname => user.lname,
    :photo => user.profile_pic.url(:small)
  }
end

My challenge is I now want to include records from the @contacts table that have a different set of fields than the User model.

I tried doing

@users = User.all
@contacts = current_user.contacts
@users << @contacts

But that did not work. What's the best way to combine two similar models into one JSON object?