Create IssuableSeverity table

IssuableSeverity table will keep the information about
an Incident severity level

app/models/issuable_severity.rb

+# frozen_string_literal: true
+
+class IssuableSeverity < ApplicationRecord
+  belongs_to :issue
+
+  validates :issue, presence: true, uniqueness: true
+  validates :severity, presence: true
+
+  enum severity: {
+    unknown: 0,
+    low: 1,
+    medium: 2,
+    high: 3,
+    critical: 4
+  }
+end

db/migrate/20200824124623_create_issuable_severities.rb

+# frozen_string_literal: true
+
+class CreateIssuableSeverities < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  def up
+    with_lock_retries do
+      create_table :issuable_severities do |t|
+        t.references :issue, index: { unique: true }, null: false, foreign_key: { on_delete: :cascade }
+        t.integer :severity, null: false, default: 0, limit: 2 # 0 - will stand for unknown
+      end
+    end
+  end
+
+  def down
+    with_lock_retries do
+      drop_table :issuable_severities
+    end
+  end
+end

db/schema_migrations/20200824124623

+b8fcbdab769758753efae992e64bed7f79149c74f08294035a48a03c59bb1c5d
\ No newline at end of file

db/structure.sql

@@ -12479,6 +12479,21 @@ CREATE SEQUENCE public.ip_restrictions_id_seq
 
 ALTER SEQUENCE public.ip_restrictions_id_seq OWNED BY public.ip_restrictions.id;
 
+CREATE TABLE public.issuable_severities (
+    id bigint NOT NULL,
+    issue_id bigint NOT NULL,
+    severity smallint DEFAULT 0 NOT NULL
+);
+
+CREATE SEQUENCE public.issuable_severities_id_seq
+    START WITH 1
+    INCREMENT BY 1
+    NO MINVALUE
+    NO MAXVALUE
+    CACHE 1;
+
+ALTER SEQUENCE public.issuable_severities_id_seq OWNED BY public.issuable_severities.id;
+
 CREATE TABLE public.issue_assignees (
     user_id integer NOT NULL,
     issue_id integer NOT NULL
@@ -17052,6 +17067,8 @@ ALTER TABLE ONLY public.internal_ids ALTER COLUMN id SET DEFAULT nextval('public
 
 ALTER TABLE ONLY public.ip_restrictions ALTER COLUMN id SET DEFAULT nextval('public.ip_restrictions_id_seq'::regclass);
 
+ALTER TABLE ONLY public.issuable_severities ALTER COLUMN id SET DEFAULT nextval('public.issuable_severities_id_seq'::regclass);
+
 ALTER TABLE ONLY public.issue_links ALTER COLUMN id SET DEFAULT nextval('public.issue_links_id_seq'::regclass);
 
 ALTER TABLE ONLY public.issue_metrics ALTER COLUMN id SET DEFAULT nextval('public.issue_metrics_id_seq'::regclass);
@@ -18132,6 +18149,9 @@ ALTER TABLE ONLY public.internal_ids
 ALTER TABLE ONLY public.ip_restrictions
     ADD CONSTRAINT ip_restrictions_pkey PRIMARY KEY (id);
 
+ALTER TABLE ONLY public.issuable_severities
+    ADD CONSTRAINT issuable_severities_pkey PRIMARY KEY (id);
+
 ALTER TABLE ONLY public.issue_links
     ADD CONSTRAINT issue_links_pkey PRIMARY KEY (id);
 
@@ -19853,6 +19873,8 @@ CREATE UNIQUE INDEX index_internal_ids_on_usage_and_project_id ON public.interna
 
 CREATE INDEX index_ip_restrictions_on_group_id ON public.ip_restrictions USING btree (group_id);
 
+CREATE UNIQUE INDEX index_issuable_severities_on_issue_id ON public.issuable_severities USING btree (issue_id);
+
 CREATE UNIQUE INDEX index_issue_assignees_on_issue_id_and_user_id ON public.issue_assignees USING btree (issue_id, user_id);
 
 CREATE INDEX index_issue_assignees_on_user_id ON public.issue_assignees USING btree (user_id);
@@ -22190,6 +22212,9 @@ ALTER TABLE ONLY public.protected_branch_unprotect_access_levels
 ALTER TABLE ONLY public.ci_freeze_periods
     ADD CONSTRAINT fk_rails_2e02bbd1a6 FOREIGN KEY (project_id) REFERENCES public.projects(id);
 
+ALTER TABLE ONLY public.issuable_severities
+    ADD CONSTRAINT fk_rails_2fbb74ad6d FOREIGN KEY (issue_id) REFERENCES public.issues(id) ON DELETE CASCADE;
+
 ALTER TABLE ONLY public.saml_providers
     ADD CONSTRAINT fk_rails_306d459be7 FOREIGN KEY (group_id) REFERENCES public.namespaces(id) ON DELETE CASCADE;
 

spec/factories/issuable_severity.rb

+# frozen_string_literal: true
+
+FactoryBot.define do
+  factory :issuable_severity do
+    association :issue, factory: :incident
+  end
+end

spec/models/issuable_severity_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe IssuableSeverity, type: :model do
+  let_it_be(:issuable_severity) { create(:issuable_severity) }
+
+  describe 'associations' do
+    it { is_expected.to belong_to(:issue) }
+  end
+
+  describe 'validations' do
+    it { is_expected.to validate_presence_of(:severity) }
+    it { is_expected.to validate_presence_of(:issue) }
+    it { is_expected.to validate_uniqueness_of(:issue) }
+  end
+
+  describe 'enums' do
+    let(:severity_values) do
+      { unknown: 0, low: 1, medium: 2, high: 3, critical: 4 }
+    end
+
+    it { is_expected.to define_enum_for(:severity).with_values(severity_values) }
+  end
+end