I have three entities : Service, Group, Agent.
Service => oneToMany => Group => oneToMany => Agent
Each of these entities already have a form. By the way, I would like to create a custom mergeForm which will contains a select field Service to choose and a select field Groupe in order to merge some of these entities. The Symfony 2 official cookbook is under maintenance, so I would like to know the way to create the custom Form in which I will be able to choose a Service (from all existing services) and to choose a Group (from all existing groups)