Default admin belongs_to relationship fails if model not registered

[augnustin]

If a model has a belongs_to relationship to another model that is not registered in config.exs, it fails with the following error:

[error] %UndefinedFunctionError{arity: 0, function: :__struct__, message: nil, module: nil, reason: nil}
[error] [{nil, :__struct__, [], []}, {ExAdmin.Helpers, :get_resource_field, 3, [file: 'lib/ex_admin/helpers.ex', line: 318]}, {Enum, :"-map/2-lists^map/1-0-", 2, [file: 'lib/enum.ex', line: 1327]}, {ExAdmin.Helpers, :map_relationship_fields, 3, [file: 'lib/ex_admin/helpers.ex', line: 100]}, {ExAdmin.Helpers, :build_single_field_type, 5, [file: 'lib/ex_admin/helpers.ex', line: 242]}, {ExAdmin.Helpers, :build_field, 4, [file: 'lib/ex_admin/helpers.ex', line: 139]}, {ExAdmin.Theme.AdminLte2.Index, :"-build_table_body/4-fun-1-", 4, [file: 'lib/ex_admin/themes/admin_lte2/index.ex', line: 265]}, {Enum, :"-reduce/3-lists^foldl/2-0-", 3, [file: 'lib/enum.ex', line: 1940]}, {ExAdmin.Theme.AdminLte2.Index, :"-build_table_body/4-fun-2-", 5, [file: 'lib/ex_admin/themes/admin_lte2/index.ex', line: 264]}, {Enum, :"-map/2-lists^map/1-0-", 2, [file: 'lib/enum.ex', line: 1327]}, {ExAdmin.Theme.AdminLte2.Index, :build_table_body, 4, [file: 'lib/ex_admin/themes/admin_lte2/index.ex', line: 246]}, {ExAdmin.Theme.AdminLte2.Index, :paginated_collection_table, 2, [file: 'lib/ex_admin/themes/admin_lte2/index.ex', line: 64]}, {ExAdmin.Theme.AdminLte2.Index, :batch_action_form, 6, [file: 'lib/ex_admin/themes/admin_lte2/index.ex', line: 226]}, {ExAdmin.Theme.AdminLte2.Index, :wrap_index_grid, 1, [file: 'lib/ex_admin/themes/admin_lte2/index.ex', line: 14]}, {ExAdmin.Index, :default_index_view, 3, [file: 'lib/ex_admin/index.ex', line: 290]}, {ExAdmin.AdminResourceController, :index, 3, [file: 'web/controllers/admin_resource_controller.ex', line: 28]}, {ExAdmin.AdminResourceController, :action, 2, [file: 'web/controllers/admin_resource_controller.ex', line: 1]}, {ExAdmin.AdminResourceController, :phoenix_controller_pipeline, 2, [file: 'web/controllers/admin_resource_controller.ex', line: 1]}, {Vae.Endpoint, :instrument, 4, [file: 'lib/vae/endpoint.ex', line: 1]}, {Phoenix.Router, :__call__, 1, [file: 'lib/phoenix/router.ex', line: 278]}]

I solved it by adding the second model to the admin, but this was not conceptually what I wanted to do because it was not supposed to be administrable.

Reporting the issue for the records.