Fix export action didn't use export section for eager loading

Fixes #1954

Author: mshibuya

app/controllers/rails_admin/main_controller.rb

@@ -123,7 +123,8 @@ def check_for_cancel
     end
 
     def get_collection(model_config, scope, pagination)
-      eager_loads = model_config.list.fields.flat_map(&:eager_load_values)
+      section = @action.key == :export ? model_config.export : model_config.list
+      eager_loads = section.fields.flat_map(&:eager_load_values)
       options = {}
       options = options.merge(page: (params[Kaminari.config.param_name] || 1).to_i, per: (params[:per] || model_config.list.items_per_page)) if pagination
       options = options.merge(include: eager_loads) unless eager_loads.blank?

spec/controllers/rails_admin/main_controller_spec.rb

@@ -10,6 +10,10 @@ def get(action, params)
     super action, params: params
   end
 
+  before do
+    controller.instance_variable_set :@action, RailsAdmin::Config::Actions.find(:index)
+  end
+
   describe '#check_for_cancel' do
     before do
       allow(controller).to receive(:back_or_index) { raise StandardError.new('redirected back') }
@@ -266,6 +270,24 @@ def get(action, params)
       expect(abstract_model).to receive(:all).with(hash_including(include: [{players: :draft}]), nil).once.and_call_original
       controller.send(:get_collection, model_config, nil, false).to_a
     end
+
+    context 'on export' do
+      before do
+        controller.instance_variable_set :@action, RailsAdmin::Config::Actions.find(:export)
+      end
+
+      it 'uses the export section' do
+        RailsAdmin.config Team do
+          export do
+            field :players do
+              eager_load true
+            end
+          end
+        end
+        expect(abstract_model).to receive(:all).with(hash_including(include: [:players]), nil).once.and_call_original
+        controller.send(:get_collection, model_config, nil, false).to_a
+      end
+    end
   end
 
   describe 'index' do