How can I make an ActiveModel::Serializer attribute optional at runtime?

Question

I am trying to allow an API request to specify what fields to return on an object. I can retrieve the object with only the fields specified, but when it is serialized, it throws an error:

ActiveModel::MissingAttributeError (missing attribute: x)

How can I achieve this functionality with ActiveModel::Serializer and is it possible?

score 6 · Answer 1 · edited May 06 '21 at 05:56

6

I've found this question while searching for a good alternative to remove optional fields from the json response.

The gem active_model_serializers does have a solution for this. You just need to pass a conditional to the attribute method in the serializer declaration.

class MySelectiveSerializer < ActiveModel::Serializer
  attributes :id, :anything
  attribute :something, if: -> { object.something.present? }
end

Perhaps 3 years ago a solution like this didn't exist, but it is available now. :)

Cheers.

edited May 06 '21 at 05:56

wscourge

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answered Aug 21 '19 at 05:15

robsonmwoc

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In what version was this introduced? Can you link to a reference? – Dennis Dec 03 '21 at 19:49

voiski · Answer 2 · 2017-11-17T14:41:34.343

This happens because the Serializer.attributes method call each field using the ActiveModel.read_attribute method. This method will apply some validations, like validates_presence_of at the model's definition, that will raise the exception. To avoid it I give three bad solutions and after a better and simple one:

Change the model definition, but you will miss your validation.
Overwrite the method ActiveModel.read_attribute to handle this behavior, you will get new challenges.
Overwrite the Serializer.attributes and instead of call super, call object.attributes.

But the best option will be create a new serialize class, to avoid besides effects, with the only fields that you want. Then specify this at the controller class:

render json: People.all.reduced, each_serializer: SimplePersonSerializer

Edit 1

The right answer should be the one from Maurício Linhares.

render json: result.to_json( only: array_of_fields )

The final suggestions doesn't meet the challenge. OP wants the API consumer to be able to specify any combination of fields rather than a specific, predefined subset. — Adamantish, Nov 16 '17 at 13:50
@Adamantish thanks for this, I didn't realize that it should be flexible, my bad =( — voiski, Nov 17 '17 at 14:43

score 2 · Answer 3 · answered Sep 09 '15 at 23:12

2

You can remove attributes from serializer, but they should exist.

class SomeSerializer < ActiveModel::Serializer
  attributes :something

  def attributes
     super.except(:something) if something
   end
end

answered Sep 09 '15 at 23:12

droptheplot

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score 0 · Answer 4 · answered Sep 10 '15 at 12:30

0

You can customize attributes by implementing filter method in your serializer. Note, that I describe latest stable (for the time of the writing this post) 0.9.x branch.

class PostSerializer < ActiveModel::Serializer
  attributes :id, :title, :body, :author

  def filter(keys)
    if scope.admin?
      keys
    else
      keys - [:author]
    end
  end
end

answered Sep 10 '15 at 12:30

Alexey Shein

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Doesn't solve the problem filter is called after the looking for all the attributes so error is still thrown. – Adamantish Nov 16 '17 at 13:47

How can I make an ActiveModel::Serializer attribute optional at runtime?

4 Answers4