Add DesignManagement::Design model

The design model will represent one single file uploaded to a
`DesignCollection` linked to an issue.

db/schema.rb

@@ -1035,6 +1035,15 @@ ActiveRecord::Schema.define(version: 20190305162221) do
     t.index ["project_id", "status"], name: "index_deployments_on_project_id_and_status", using: :btree
   end
 
+  create_table "design_management_designs", id: :bigserial, force: :cascade do |t|
+    t.integer "project_id", null: false
+    t.integer "issue_id", null: false
+    t.string "filename", null: false
+    t.index ["issue_id", "filename"], name: "index_design_management_designs_on_issue_id_and_filename", unique: true, using: :btree
+    t.index ["issue_id"], name: "index_design_management_designs_on_issue_id", unique: true, using: :btree
+    t.index ["project_id"], name: "index_design_management_designs_on_project_id", using: :btree
+  end
+
   create_table "draft_notes", id: :bigserial, force: :cascade do |t|
     t.integer "merge_request_id", null: false
     t.integer "author_id", null: false
@@ -3405,6 +3414,8 @@ ActiveRecord::Schema.define(version: 20190305162221) do
   add_foreign_key "container_repositories", "projects"
   add_foreign_key "deploy_keys_projects", "projects", name: "fk_58a901ca7e", on_delete: :cascade
   add_foreign_key "deployments", "projects", name: "fk_b9a3851b82", on_delete: :cascade
+  add_foreign_key "design_management_designs", "issues", on_delete: :cascade
+  add_foreign_key "design_management_designs", "projects", on_delete: :cascade
   add_foreign_key "draft_notes", "merge_requests", on_delete: :cascade
   add_foreign_key "draft_notes", "users", column: "author_id", on_delete: :cascade
   add_foreign_key "environments", "projects", name: "fk_d1c8c1da6a", on_delete: :cascade

ee/app/models/design_management.rb

+# frozen_string_literal: true
+
+module DesignManagement
+  def self.table_name_prefix
+    'design_management_'
+  end
+end

ee/app/models/design_management/design.rb

+# frozen_string_literal: true
+
+module DesignManagement
+  class Design < ApplicationRecord
+    belongs_to :project
+    belongs_to :issue
+    has_many :versions, class_name: 'DesignManagement::Version', inverse_of: :design
+
+    validates :project, :issue, :filename, presence: true
+    validates :issue, uniqueness: true
+    validates :filename, uniqueness: { scope: :issue_id }
+  end
+end

ee/db/migrate/20190228134845_create_designs.rb

+class CreateDesigns < ActiveRecord::Migration[5.0]
+  def change
+    create_table :design_management_designs, id: :bigserial do |t|
+      t.references :project, foreign_key: { on_delete: :cascade }, index: true, null: false
+      t.references :issue, foreign_key: { on_delete: :cascade }, index: { unique: true }, null: false
+
+      t.string :filename, null: false
+      t.index [:issue_id, :filename], unique: true
+    end
+  end
+end

ee/spec/factories/design_management/designs.rb

+# frozen_string_literal: true
+
+FactoryBot.define do
+  factory :design, class: DesignManagement::Design do
+    issue
+    project { issue.project }
+    sequence(:filename) { |n| "homescreen-#{n}.jpg" }
+  end
+end

ee/spec/models/design_management/design_spec.rb

+# frozen_string_literal: true
+
+require 'rails_helper'
+
+describe DesignManagement::Design do
+  describe 'relations' do
+    it { is_expected.to belong_to(:project) }
+    it { is_expected.to belong_to(:issue) }
+    it { is_expected.to have_many(:versions) }
+  end
+
+  describe 'validations' do
+    subject(:design) { build(:design) }
+
+    it { is_expected.to be_valid }
+    it { is_expected.to validate_presence_of(:project) }
+    it { is_expected.to validate_presence_of(:issue) }
+    it { is_expected.to validate_presence_of(:filename) }
+    it { is_expected.to validate_uniqueness_of(:issue) }
+    it { is_expected.to validate_uniqueness_of(:filename).scoped_to(:issue_id) }
+  end
+end