Apirest calls unknown foreign key

I am new in GLPI and I have to create a plugin that I query with the API provided by GLPI. By consulting the example plugin I therefore created a first table 'plugin_toip_installations' with foreign keys such as 'entities_id' linked to the GLPI's table and 'plugin_toip_foreignkeys_id' linked to my second table 'plugin_toip_foreignkeys'. I create 'PluginToipInstallation' class extends CommonDBTM and 'PluginToipForeignkey' class extends CommonDropdown. When I request api I see my relationship as 'UNKNOWN' for 'foreignkeys_id' and with the option expand_dropdown the data is empty, not for 'entities_id'. What I missed ? Thanks for your help.