The API isn't using the appropriate services for managing forks

app/services/projects/fork_service.rb

 module Projects
   class ForkService < BaseService
-    def execute
+    def execute(fork_to_project = nil)
+      if fork_to_project
+        link_existing_project(fork_to_project)
+      else
+        fork_new_project
+      end
+    end
+
+    private
+
+    def link_existing_project(fork_to_project)
+      return if fork_to_project.forked?
+
+      link_fork_network(fork_to_project)
+
+      fork_to_project
+    end
+
+    def fork_new_project
       new_params = {
         forked_from_project_id: @project.id,
         visibility_level:       allowed_visibility_level,
@@ -21,15 +39,11 @@ module Projects
       builds_access_level = @project.project_feature.builds_access_level
       new_project.project_feature.update_attributes(builds_access_level: builds_access_level)
 
-      refresh_forks_count
-
       link_fork_network(new_project)
 
       new_project
     end
 
-    private
-
     def fork_network
       if @project.fork_network
         @project.fork_network
@@ -43,9 +57,17 @@ module Projects
       end
     end
 
-    def link_fork_network(new_project)
-      fork_network.fork_network_members.create(project: new_project,
+    def link_fork_network(fork_to_project)
+      fork_network.fork_network_members.create(project: fork_to_project,
                                                forked_from_project: @project)
+
+      # TODO: remove this when ForkedProjectLink model is removed
+      unless fork_to_project.forked_project_link
+        fork_to_project.create_forked_project_link(forked_to_project: fork_to_project,
+                                                   forked_from_project: @project)
+      end
+
+      refresh_forks_count
     end
 
     def refresh_forks_count

changelogs/unreleased/fj-40752-forks-api-not-using-services.yml

+---
+title: Using appropiate services in the API for managing forks
+merge_request: 15709
+author:
+type: fixed

db/post_migrate/20171205190711_reschedule_fork_network_creation_caller.rb

+class RescheduleForkNetworkCreationCaller < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  MIGRATION = 'PopulateForkNetworksRange'.freeze
+  BATCH_SIZE = 100
+  DELAY_INTERVAL = 15.seconds
+
+  disable_ddl_transaction!
+
+  class ForkedProjectLink < ActiveRecord::Base
+    include EachBatch
+
+    self.table_name = 'forked_project_links'
+  end
+
+  def up
+    say 'Populating the `fork_networks` based on existing `forked_project_links`'
+
+    queue_background_migration_jobs_by_range_at_intervals(ForkedProjectLink, MIGRATION, DELAY_INTERVAL, batch_size: BATCH_SIZE)
+  end
+
+  def down
+    # nothing
+  end
+end

db/schema.rb

@@ -11,7 +11,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 20171124150326) do
+ActiveRecord::Schema.define(version: 20171205190711) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"

lib/api/projects.rb

@@ -367,15 +367,16 @@ module API
       post ":id/fork/:forked_from_id" do
         authenticated_as_admin!
 
-        forked_from_project = find_project!(params[:forked_from_id])
-        not_found!("Source Project") unless forked_from_project
+        fork_from_project = find_project!(params[:forked_from_id])
 
-        if user_project.forked_from_project.nil?
-          user_project.create_forked_project_link(forked_to_project_id: user_project.id, forked_from_project_id: forked_from_project.id)
+        not_found!("Source Project") unless fork_from_project
 
-          ::Projects::ForksCountService.new(forked_from_project).refresh_cache
+        result = ::Projects::ForkService.new(fork_from_project, current_user).execute(user_project)
+
+        if result
+          present user_project.reload, with: Entities::Project
         else
-          render_api_error!("Project already forked", 409)
+          render_api_error!("Project already forked", 409) if user_project.forked?
         end
       end
 
@@ -383,11 +384,11 @@ module API
       delete ":id/fork" do
         authorize! :remove_fork_project, user_project
 
-        if user_project.forked?
-          destroy_conditionally!(user_project.forked_project_link)
-        else
-          not_modified!
+        result = destroy_conditionally!(user_project) do
+          ::Projects::UnlinkForkService.new(user_project, current_user).execute
         end
+
+        result ? status(204) : not_modified!
       end
 
       desc 'Share the project with a group' do