Allow serialized ID to be overwritten for belongs-to relationships

[greysteil]

Purpose

If the id attribute for a class isn't taken directly from the object when serializing it, it may be desirable for other classes that serialize a relationship with that class to overwrite the relationship IDs they serialize.

For example, suppose we have:

class Repo < Model
  attributes :id, :github_id, :name
  associations :configs
end

class Config < Model
  attributes :id
  belongs_to :repo
end

class RepoSerializer < ActiveModel::Serializer
  attributes :id, :name

  has_many :update_configs

  def id
    object.github_id
  end
end

class ConfigSerializer < ActiveModel::Serializer
  attributes :id
  belongs_to :repo
end

In the above example, serializing a list of Repos will give the github_id for each one, but serializing a Config will give the id for its parent repo.

Changes

I've updated the belongs_to? path of Relationship#data_for_one to use read_attribute_for_serialization instead of object.send to fetch the serialized foreign key. This allows the serialized relationship ID to be
overwritten using:

class ConfigSerializer < ActiveModel::Serializer
  ...

  def repo_id
    object.repo.github_id
  end
end

Caveats

Ideally AMS would inspect the RepoSerializer when serializing the Config, and realise it can't just output the foreign key. Unfortunately, getting the serialization class for the child repo currently requires loading the record (via evaluating lazy_assocation), and loses the performance benefit of the existing belongs_to? path.

Related GitHub issues

#2125. (Not resolved by this, but the ID issue is at least improved, for no performance cost.)

Additional helpful information

[mention-bot]

@greysteil, thanks for your PR! By analyzing the history of the files in this pull request, we identified @bf4, @beauby and @bolshakov to be potential reviewers.

[bf4]

Looks good. Can you add an entry to the changelog?

[greysteil]

@bf4 - done.

[bf4]

@greysteil Thanks!