Polymorphic ResourceOwner support (https://github.com/doorkeeper-gem/doorkeeper/issues/651#issuecomment-99880425)

access_token.rb

# app/models/doorkeeper/access_token.rb

module Doorkeeper
  class AccessToken < ActiveRecord::Base
    belongs_to :resource_owner, class_name: 'OauthResourceOwner'

    def as_json(*)
      super.except!(:resource_owner_id).merge!(
        owner: {
          id: resource_owner&.owner_id,
          type: resource_owner&.owner_type&.tableize
        }
      )
    end
  end
end

doorkeeper.rb

# config/doorkeeper.rb

Doorkeeper.configure do
  orm :active_record

  # ...

  resource_owner_from_credentials do |_routes|
    app = Doorkeeper::Application.by_uid(params[:client_id])
    OauthResourceOwnerAuthenticator.authenticate(app, request) if app
  end
  
  # ...
end

HTTP.md

POST /oauth/token HTTP/1.1
Content-Type: application/json

{
  "grant_type": "password",
  "client_id": "CLIENT_ID",
  "owner_type": "OWNER_TYPE",
  "username": "OWNER_EMAIL",
  "password": "OWNER_PASSWORD"
}

HTTP/1.1 200 OK
Content-Type: application/json; charset=utf-8

{
  "access_token": "e2892936f4...3a0d3171b0",
  "token_type": "bearer",
  "expires_in": 7200,
  "refresh_token": "6231777801...735af7da27",
  "created_at": 1430921359,
  "owner_id": "OWNER_ID",
  "owner_type": "OWNER_TYPE"
}

oauth_resource_owner.rb

# app/models/oauth_resource_owner.rb

class OauthResourceOwner < ActiveRecord::Base
  belongs_to :owner, polymorphic: true
end

oauth_resource_owner_authenticator.rb

# app/services/oauth_resource_owner_authenticator.rb

class OauthResourceOwnerAuthenticator
  attr_reader :application, :request
  delegate :params, to: :request

  def self.authenticate(*args)
    new(*args).authenticate
  end

  def initialize(application, request)
    @application = application
    @request = request
  end

  def authenticate
    return unless owner_class
    
    owner = authenticate_owner
    OauthResourceOwner.find_or_create_by(owner: owner) if owner
  end

  private

  def authenticate_owner
    owner = owner_class.find_for_database_authentication(
      email: params[:username]
    )
    return unless owner&.valid_password?(params[:password])

    update_tracked_fields(owner)
    owner
  end
  
  # Update Devise tracked fields
  def update_tracked_fields(owner)
    if owner.respond_to?(:update_tracked_fields!)
      owner.update_tracked_fields!(request)
    end
  end

  def owner_class
    case params[:owner_type]
    when /admin/i then Admin
    when /user/i then User
    end
  end
end

token_response_body_with_resource_owner.rb

# lib/doorkeeper/oauth/token_response_body_with_resource_owner.rb

module Doorkeeper
  module OAuth
    module TokenResponseBodyWithResourceOwner
      def body
        super.merge({
          'owner_id'   => token.resource_owner&.owner_id,
          'owner_type' => token.resource_owner&.owner_type&.tableize
        }.compact)
      end
    end

    TokenResponse.prepend(TokenResponseBodyWithResourceOwner)
  end
end