Adds fixed items model/assoc to activerecord-gitlab gem

What does this MR do and why?

Adds fixed items model and associations to activerecord-gitlab gem

Model concern allows classes with fixed items to have active record like methods (all, where, find_by, find).

The has one association can be used on models to establish an association to a fixed items model.

This is an extract from the custom statuses for work items POC where we'll use fixed items models to host system defined statuses and lifecycles.

1. Draft: POC Static default status for work items (!178180 - closed)
2. See the last refactor to move this to the gem in the POC
3. Usage on WorkItems::Statuses::SystemDefined::Status in POC
4. Usage of the association in WorkItems::Statuses::CurrentStatus in POC

Contributes to BE: Create tables to support Status widget (#498393 - closed)

Examples

class StaticModel
  include ActiveModel::Model
  include ActiveModel::Attributes
  include ActiveRecord::FixedItemsModel::Model

  ITEMS = [
    {
      id: 1,
      name: 'To do'
    }
  ]

  attribute :id, :integer
  attribute :name, :string
end

StaticModel.find(1)
StaticModel.where(name: 'To do')
StaticModel.find_by(name: 'To do')
StaticModel.all

class MyModel < ApplicationRecord
  include ActiveRecord::FixedItemsModel::HasOne

  belongs_to_fixed_items :static_model, fixed_items_class: StaticModel
end

m = MyModel.last
m.static_model # Returns fixed items model instance
m.static_model = StaticModel.find(1)
m.static_model_id = 1 # still possible
m.static_model? # Bool

References

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• BE: Create tables to support Status widget (#498393 - closed)

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Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Run the specs locally:

1. Go to gem folder cd gems/activerecord-gitlab
2. bundle install
3. bundle exec rspec