Add support for eagerly loading associations via the include param

[NullVoxPopuli]

Similar to ActiveModelSerializer's include option, but for eagerly loading relationships, rather than just rendering them.

For example, instead of having to write this:

    render json: Level.includes(
      :attendances => [
        :housing_request,
        :housing_provision,
        :orders,
        :package,
        :attendee
      ]).find(params[:id]), include: params[:include]

we want the ability to have whatever is in the include param eagerly load the relationships for us upon model resolution so that our controller actions can remain as

    render json: model, include: params[:include]

[NullVoxPopuli]

See: https://github.com/rails-api/active_model_serializers/blob/master/lib/active_model/serializer/include_tree.rb for converting the string to a hash.

---

• Do we want to silently fail if an association is spelled wrong, and just output a warning to the log?
• Should we call the AMS include_tree module directly, or copy it to this repo?
  • if we copy it to this repo, then we can do some validation / name fixing against the ActiveRecord relationships

[NullVoxPopuli]

ref: rails-api/active_model_serializers#1044 (comment)

[NullVoxPopuli]

Actually going to close this, thanks to the https://github.com/salsify/goldiloader gem :-)

Thanks @salsify!