How to customize json response with active-model-serializers?

Question

I have an API, built on Rails 4.2.6, and using active-model-serializers 0.10.0. While ams is great, I want to return a key and value that don't represent columns on the table. The API is used to create a letter, and I need to return the url that the user can find that letter at. Something along the lines of:

https://{DOMAIN}.com/letters/{LETTER.ID}

Do I modify the serializer somehow to do this, or do I render some custom JSON that includes the serializer information in it? I've checked a couple of other questions that have to do with custom JSON output, but they don't cover this question specifically.

EDIT

I tried your method below, @max, although I'm not sure it's quite what you meant. Here is what I added to my serializer:

attributes :id, :url, :date_created, etc...
def url
    "https://{www.example.com/previewpdf/mailings/#{id}"
end

I tried both this, and using self.id, but both return an "undefined method 'id'" error, so I'll keep plugging away.

EDIT #2 I switched to using :id instead of id, and that made the undefined method error disappear, but the url is showing up in the JSON as

https://www.example.com/pdfpreview?SubOrderId=id

When I need that last part to read SubOrderId=28291, or whatever the id is of the mailing being created.

EDIT

This is still perplexing me, does anyone know how to include the actual value of the id key in a custom string response?

Answer 1

If a method with the same name as an attribute exists the serializer will call the method instead of getting the attribute directly from the object being serialized.

class FooSerializer < ActiveModel::Serializer
  attributes :bar, :url
  def url
    something_url(object)
  end
end

Note that the object being serialized does not need to have a url attribute.

Another way to do this is by using attribute with a block:

class FooSerializer < ActiveModel::Serializer
  attributes :bar, :url
  attribute(:url) { something_url(object) }
end