A Little Rails Knowledge (goes a long way)

CloudForms/ManageIQ is a Ruby on Rails application, and so the Automation Engine is created using Rails, but it cleverly hides most of the "Rails-ness" of the larger application from us. It is still useful however to have some understanding of a few of the features and concepts of Rails.

Model-View-Controller

Rails is a Model-View-Controller (MVC) application (see also Ruby on Rails/Getting Started/Model-View-Controller)

The Model is a represention of the underlying, logical structure of data from the database (which in the case of CloudForms/ManageIQ is PostgreSQL). When writing automation scripts, we work with models extensively (although we may not necessarily realise it).

Rails Models are called Active Records. They always have a singular CamelCase name (e.g. GuestApplication), and their corresponding database tables have a plural snake_case name (e.g. guest_applications).

Active Record Associations

Active Record Associations link the Models together in one-to-many and one-to-one relationships that allow us to traverse objects.

We can illustrate this by looking at some of the Rails code that defines the Host Active Record:

class Host < ActiveRecord::Base
  ...
  belongs_to                :ext_management_system, :foreign_key => "ems_id"
  belongs_to                :ems_cluster
  has_one                   :operating_system, :dependent => :destroy
  has_one                   :hardware, :dependent => :destroy
  has_many                  :vms_and_templates, :dependent => :nullify
  has_many                  :vms
  ...

We see that there are several associations from a host object, including to the cluster that it's a member of, and to the VMs that run on that host.

Although these associations are defined in Rails, they are available to us when we work with the corresponding MiqAeService objects from the Automation Engine (see The MiqAeService* Model).

Rails Helper Methods (.find_by_*)

Rails does a lot of things to make our lives easier, including dynamically creating helper methods. The most useful ones are the find_by_* methods.

owner = $evm.vmdb('user').find_by_id( ownerid.to_i )
vm = $evm.vmdb('vm').find_by_name(vm_name)
vm = $evm.vmdb('vm').find_by_guid(guid)

We can .find_by_ any table heading on a database table, so if we look at the services column:

vmdb_production=# \d services
                                          Table "public.services"
        Column        |            Type             |                       Modifiers
----------------------+-----------------------------+-------------------------------------------------------
 id                   | bigint                      | not null default nextval('services_id_seq'::regclass)
 name                 | character varying(255)      |
 description          | character varying(255)      |
 guid                 | character varying(255)      |
 type                 | character varying(255)      |
 service_template_id  | bigint                      |
 options              | text                        |
 display              | boolean
 ...

We see that if we wanted we could call:

$evm.vmdb('service').find_by_description('My New Service')

Tip - don't try searching the CloudForms sources for def find_by_id though, these are not statically defined methods and so don't exist in the CloudForms code. The find_by helpers are also being deprecated in Rails in favour of a syntax using where, for example: $evm.vmdb('service').where(:description => 'My New Service')

A Little Rails Knowledge