Add a model for `fork_networks`

The fork network will keep track of the root project as long as it's
present.

app/models/fork_network.rb

+class ForkNetwork < ActiveRecord::Base
+  belongs_to :root_project
+end

db/migrate/20170928124105_create_fork_networks.rb

+class CreateForkNetworks < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    create_table :fork_networks do |t|
+      t.references :root_project,
+                   references: :projects,
+                   index: { unique: true }
+
+      t.string :deleted_root_project_name
+    end
+
+    add_concurrent_foreign_key :fork_networks, :projects,
+                               column: :root_project_id,
+                               on_delete: :nullify
+  end
+
+  def down
+    if foreign_keys_for(:fork_networks, :root_project_id).any?
+      remove_foreign_key :fork_networks, column: :root_project_id
+    end
+    drop_table :fork_networks
+  end
+end

db/schema.rb

@@ -642,6 +642,13 @@ ActiveRecord::Schema.define(version: 20171002105019) do
 
   add_index "features", ["key"], name: "index_features_on_key", unique: true, using: :btree
 
+  create_table "fork_networks", force: :cascade do |t|
+    t.integer "root_project_id"
+    t.string "deleted_root_project_name"
+  end
+
+  add_index "fork_networks", ["root_project_id"], name: "index_fork_networks_on_root_project_id", unique: true, using: :btree
+
   create_table "forked_project_links", force: :cascade do |t|
     t.integer "forked_to_project_id", null: false
     t.integer "forked_from_project_id", null: false
@@ -2098,6 +2105,7 @@ ActiveRecord::Schema.define(version: 20171002105019) do
   add_foreign_key "environments", "projects", name: "fk_d1c8c1da6a", on_delete: :cascade
   add_foreign_key "events", "projects", on_delete: :cascade
   add_foreign_key "events", "users", column: "author_id", name: "fk_edfd187b6f", on_delete: :cascade
+  add_foreign_key "fork_networks", "projects", column: "root_project_id", name: "fk_e7b436b2b5", on_delete: :nullify
   add_foreign_key "forked_project_links", "projects", column: "forked_to_project_id", name: "fk_434510edb0", on_delete: :cascade
   add_foreign_key "geo_event_log", "geo_repositories_changed_events", column: "repositories_changed_event_id", name: "fk_4a99ebfd60", on_delete: :cascade
   add_foreign_key "geo_event_log", "geo_repository_created_events", column: "repository_created_event_id", name: "fk_9b9afb1916", on_delete: :cascade

spec/lib/gitlab/import_export/all_models.yml

@@ -293,6 +293,9 @@ project:
 - source_pipelines
 - sourced_pipelines
 - members_and_requesters
+- root_of_fork_network
+- fork_network_member
+- fork_network
 award_emoji:
 - awardable
 - user

spec/models/fork_network_spec.rb

+require 'rails_helper'
+
+RSpec.describe ForkNetwork, type: :model do
+  pending "add some examples to (or delete) #{__FILE__}"
+end