Conditional attributes in Active Model Serializers

Question

How do I render an attribute only if some condition is true?

For example, I want to render User's token attribute on create action.

Answer 1

You can also do it this way:

class EntitySerializer < ActiveModel::Serializer
  attributes :id, :created_at, :updated_at
  attribute :conditional_attr, if: :condition?

  def condition?
    #condition code goes here
  end
end

For example:

class UserSerializer < ActiveModel::Serializer
  attributes :id, :username, :name, :email, :created_at, :updated_at
  attribute :auth_token, if: :auth_token?

  def created_at
    object.created_at.to_i
  end

  def updated_at
    object.updated_at.to_i
  end

  def auth_token?
    true if object.auth_token
  end
end

EDIT (Suggested by Joe Essey) :

This method doesn't work with latest version (0.10)

With the version 0.8 it is even simpler. You don't have to use the if: :condition?. Instead you can use the following convention to achieve the same result.

class EntitySerializer < ActiveModel::Serializer
  attributes :id, :created_at, :updated_at
  attribute :conditional_attr

  def include_conditional_attr?
    #condition code goes here
  end
end

The example above would look like this.

class UserSerializer < ActiveModel::Serializer
  attributes :id, :username, :name, :email, :created_at, :updated_at
  attribute :auth_token

  def created_at
    object.created_at.to_i
  end

  def updated_at
    object.updated_at.to_i
  end

  def include_auth_token?
    true if object.auth_token
  end
end

See 0.8 documentation for more details.

Answer 2

you can override the attributes method, here is a simple example:

class Foo < ActiveModel::Serializer

  attributes :id

  def attributes(*args)
    hash = super
    hash[:last_name] = 'Bob' unless object.persisted?
    hash
  end

end

Answer 3

You could start by setting a condition on the serializers 'initialize' method. This condition can be passed from wherever else in your code, included in the options hash that 'initialize' accepts as second argument:

class SomeCustomSerializer < ActiveModel::Serializer

  attributes :id, :attr1, :conditional_attr2, :conditional_attr2

  def initialize(object, options={})
    @condition = options[:condition].present? && options[:condition]
    super(object, options)
  end

  def attributes(*args)
    return super unless @condition  #get all the attributes
    attributes_to_remove = [:conditional_attr2, :conditional_attr2]
    filtered = super.except(*attributes_to_remove)
    filtered
  end
end

In this case attr1 would always be passed, while the conditional attributes would be hidden if the condition is true.

You would get the result of this custom serialization wherever else in your code as follows:

custom_serialized_object = SomeCustomSerializer.new(object_to_serialize, {:condition => true})

I hope this was useful!

Answer 4

Serializer options were merged into ActiveModel Serializers and now are available (since 0.10).

Answer 5

Override is a good idea, but if you use the super the attributes will be calculated before you remove what you want. If it does not make difference to you, ok, but when it does, you can use it:

def attributes(options={})
  attributes =
    if options[:fields]
      self.class._attributes & options[:fields]
    else
      self.class._attributes.dup
    end

  attributes.delete_if {|attr| attr == :attribute_name } if condition

  attributes.each_with_object({}) do |name, hash|
    unless self.class._fragmented
      hash[name] = send(name)
    else
      hash[name] = self.class._fragmented.public_send(name)
    end
  end
end

ps: v0.10.0.rc3