Create RelatedEpic table and model

Create table to store related epics records
with indexes.
Add RelatedEpic model, factory and tests

Changelog: added

db/migrate/20220211125954_create_related_epic_links.rb

+# frozen_string_literal: true
+
+class CreateRelatedEpicLinks < Gitlab::Database::Migration[1.0]
+  def up
+    create_table :related_epic_links do |t|
+      t.references :source, index: true, foreign_key: { to_table: :epics, on_delete: :cascade }, null: false
+      t.references :target, index: true, foreign_key: { to_table: :epics, on_delete: :cascade }, null: false
+      t.integer :link_type, null: false, default: 0, limit: 2
+      t.timestamps_with_timezone null: false
+
+      t.index [:source_id, :target_id], unique: true
+    end
+  end
+
+  def down
+    drop_table :related_epic_links
+  end
+end

db/schema_migrations/20220211125954

+73feefe409b9c0f4ea373d0c3f13690df0086fbc4fc212595e959ad65fcc27b1
\ No newline at end of file

db/structure.sql

@@ -19197,6 +19197,24 @@ CREATE SEQUENCE redirect_routes_id_seq
 
 ALTER SEQUENCE redirect_routes_id_seq OWNED BY redirect_routes.id;
 
+CREATE TABLE related_epic_links (
+    id bigint NOT NULL,
+    source_id bigint NOT NULL,
+    target_id bigint NOT NULL,
+    link_type smallint DEFAULT 0 NOT NULL,
+    created_at timestamp with time zone NOT NULL,
+    updated_at timestamp with time zone NOT NULL
+);
+
+CREATE SEQUENCE related_epic_links_id_seq
+    START WITH 1
+    INCREMENT BY 1
+    NO MINVALUE
+    NO MAXVALUE
+    CACHE 1;
+
+ALTER SEQUENCE related_epic_links_id_seq OWNED BY related_epic_links.id;
+
 CREATE TABLE release_links (
     id bigint NOT NULL,
     release_id integer NOT NULL,
@@ -22280,6 +22298,8 @@ ALTER TABLE ONLY raw_usage_data ALTER COLUMN id SET DEFAULT nextval('raw_usage_d
 
 ALTER TABLE ONLY redirect_routes ALTER COLUMN id SET DEFAULT nextval('redirect_routes_id_seq'::regclass);
 
+ALTER TABLE ONLY related_epic_links ALTER COLUMN id SET DEFAULT nextval('related_epic_links_id_seq'::regclass);
+
 ALTER TABLE ONLY release_links ALTER COLUMN id SET DEFAULT nextval('release_links_id_seq'::regclass);
 
 ALTER TABLE ONLY releases ALTER COLUMN id SET DEFAULT nextval('releases_id_seq'::regclass);
@@ -24204,6 +24224,9 @@ ALTER TABLE ONLY raw_usage_data
 ALTER TABLE ONLY redirect_routes
     ADD CONSTRAINT redirect_routes_pkey PRIMARY KEY (id);
 
+ALTER TABLE ONLY related_epic_links
+    ADD CONSTRAINT related_epic_links_pkey PRIMARY KEY (id);
+
 ALTER TABLE ONLY release_links
     ADD CONSTRAINT release_links_pkey PRIMARY KEY (id);
 
@@ -27616,6 +27639,12 @@ CREATE UNIQUE INDEX index_redirect_routes_on_path_unique_text_pattern_ops ON red
 
 CREATE INDEX index_redirect_routes_on_source_type_and_source_id ON redirect_routes USING btree (source_type, source_id);
 
+CREATE INDEX index_related_epic_links_on_source_id ON related_epic_links USING btree (source_id);
+
+CREATE UNIQUE INDEX index_related_epic_links_on_source_id_and_target_id ON related_epic_links USING btree (source_id, target_id);
+
+CREATE INDEX index_related_epic_links_on_target_id ON related_epic_links USING btree (target_id);
+
 CREATE UNIQUE INDEX index_release_links_on_release_id_and_name ON release_links USING btree (release_id, name);
 
 CREATE UNIQUE INDEX index_release_links_on_release_id_and_url ON release_links USING btree (release_id, url);
@@ -30239,6 +30268,9 @@ ALTER TABLE ONLY packages_debian_group_distributions
 ALTER TABLE ONLY packages_conan_file_metadata
     ADD CONSTRAINT fk_rails_0afabd9328 FOREIGN KEY (package_file_id) REFERENCES packages_package_files(id) ON DELETE CASCADE;
 
+ALTER TABLE ONLY related_epic_links
+    ADD CONSTRAINT fk_rails_0b72027748 FOREIGN KEY (target_id) REFERENCES epics(id) ON DELETE CASCADE;
+
 ALTER TABLE ONLY ci_build_pending_states
     ADD CONSTRAINT fk_rails_0bbbfeaf9d FOREIGN KEY (build_id) REFERENCES ci_builds(id) ON DELETE CASCADE;
 
@@ -31367,6 +31399,9 @@ ALTER TABLE ONLY group_deploy_keys_groups
 ALTER TABLE ONLY merge_request_user_mentions
     ADD CONSTRAINT fk_rails_c440b9ea31 FOREIGN KEY (note_id) REFERENCES notes(id) ON DELETE CASCADE;
 
+ALTER TABLE ONLY related_epic_links
+    ADD CONSTRAINT fk_rails_c464534def FOREIGN KEY (source_id) REFERENCES epics(id) ON DELETE CASCADE;
+
 ALTER TABLE ONLY boards_epic_board_recent_visits
     ADD CONSTRAINT fk_rails_c4dcba4a3e FOREIGN KEY (group_id) REFERENCES namespaces(id) ON DELETE CASCADE;
 

ee/app/models/epic/related_epic_link.rb

+# frozen_string_literal: true
+
+class Epic::RelatedEpicLink < ApplicationRecord
+  self.table_name = 'related_epic_links'
+
+  belongs_to :source, class_name: 'Epic'
+  belongs_to :target, class_name: 'Epic'
+
+  validates :source, presence: true
+  validates :target, presence: true
+  validates :source, uniqueness: { scope: :target_id, message: 'is already related' }
+  validate :check_self_relation
+  validate :check_opposite_relation
+
+  scope :for_source_epic, ->(epic) { where(source_id: epic.id) }
+  scope :for_target_epic, ->(epic) { where(target_id: epic.id) }
+
+  TYPE_RELATES_TO = 'relates_to'
+  TYPE_BLOCKS = 'blocks'
+  TYPE_IS_BLOCKED_BY = 'is_blocked_by'
+
+  enum link_type: { TYPE_RELATES_TO => 0, TYPE_BLOCKS => 1 }
+
+  private
+
+  def check_self_relation
+    return unless source && target
+
+    if source == target
+      errors.add(:source, 'cannot be related to itself')
+    end
+  end
+
+  def check_opposite_relation
+    return unless source && target
+
+    if Epic::RelatedEpicLink.find_by(source: target, target: source)
+      errors.add(:source, 'is already related to this epic')
+    end
+  end
+end

ee/spec/factories/related_epic_links.rb

+# frozen_string_literal: true
+
+FactoryBot.define do
+  factory :related_epic_link, class: 'Epic::RelatedEpicLink' do
+    source factory: :epic
+    target factory: :epic
+  end
+end

ee/spec/models/epic/related_epic_link_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Epic::RelatedEpicLink do
+  describe 'Associations' do
+    it { is_expected.to belong_to(:source).class_name('Epic') }
+    it { is_expected.to belong_to(:target).class_name('Epic') }
+  end
+
+  describe 'link_type' do
+    it { is_expected.to define_enum_for(:link_type).with_values(relates_to: 0, blocks: 1) }
+
+    it 'provides the "related" as default link_type' do
+      expect(create(:related_epic_link).link_type).to eq 'relates_to'
+    end
+  end
+
+  describe 'Validation' do
+    subject { create :related_epic_link }
+
+    it { is_expected.to validate_presence_of(:source) }
+    it { is_expected.to validate_presence_of(:target) }
+    it do
+      is_expected.to validate_uniqueness_of(:source)
+                       .scoped_to(:target_id)
+                       .with_message(/already related/)
+    end
+
+    it 'is not valid if an opposite link already exists' do
+      related_epic_link = build(:related_epic_link, source: subject.target, target: subject.source)
+
+      expect(related_epic_link).to be_invalid
+      expect(related_epic_link.errors[:source]).to include('is already related to this epic')
+    end
+
+    context 'when it relates to itself' do
+      let(:epic) { create :epic }
+
+      context 'cannot be validated' do
+        it 'does not invalidate object with self relation error' do
+          related_epic_link = build(:related_epic_link, source: epic, target: nil)
+
+          related_epic_link.valid?
+
+          expect(related_epic_link.errors[:source]).to be_empty
+        end
+      end
+
+      context 'can be invalidated' do
+        it 'invalidates object' do
+          related_epic_link = build(:related_epic_link, source: epic, target: epic)
+
+          expect(related_epic_link).to be_invalid
+          expect(related_epic_link.errors[:source]).to include('cannot be related to itself')
+        end
+      end
+    end
+  end
+
+  describe 'Scopes' do
+    let_it_be(:epic1) { create(:epic) }
+    let_it_be(:epic2) { create(:epic) }
+
+    describe '.for_source_epic' do
+      it 'includes related epics for source epic' do
+        source_epic = create(:epic)
+        related_epic_link_1 = create(:related_epic_link, source: source_epic, target: epic1)
+        related_epic_link_2 = create(:related_epic_link, source: source_epic, target: epic2)
+
+        result = described_class.for_source_epic(source_epic)
+
+        expect(result).to contain_exactly(related_epic_link_1, related_epic_link_2)
+      end
+    end
+
+    describe '.for_target_epic' do
+      it 'includes related epics for target epic' do
+        target_epic = create(:epic)
+        related_epic_link_1 = create(:related_epic_link, source: epic1, target: target_epic)
+        related_epic_link_2 = create(:related_epic_link, source: epic2, target: target_epic)
+
+        result = described_class.for_target_epic(target_epic)
+
+        expect(result).to contain_exactly(related_epic_link_1, related_epic_link_2)
+      end
+    end
+  end
+end

lib/gitlab/database/gitlab_schemas.yml

@@ -440,6 +440,7 @@ push_event_payloads: :gitlab_main
 push_rules: :gitlab_main
 raw_usage_data: :gitlab_main
 redirect_routes: :gitlab_main
+related_epic_links: :gitlab_main
 release_links: :gitlab_main
 releases: :gitlab_main
 remote_mirrors: :gitlab_main