Make #session raise error when store is not configured

Add more explanation on sessions in readme

README.md

@@ -199,12 +199,33 @@ end
 
 #### Session
 
-There is support for sessions using `ActiveSupport::Cache` stores.
+This API is very close to ActiveController's session API, but works different
+under the hood. Cookies can not be used to store session id or
+whole session (like CookieStore does). So it uses key-value store and `session_key`
+method to build identifier from update.
+
+Store can be one of numerous `ActiveSupport::Cache` stores.
+While `:file_store` is suitable for development and single-server deployments
+without heavy load, it doesn't scale well. Key-value databases with persistance
+like Redis are more appropriate for production use.
 
 ```ruby
-# configure store in env files:
+# In rails app store can be configured in env files:
 config.telegram_updates_controller.session_store = :redis_store, {expires_in: 1.month}
 
+# In other app it can be done for all controllers with:
+Telegram::Bot::UpdatesController.session_store = :redis_store, {expires_in: 1.month}
+# or for specific one:
+OneOfUpdatesController.session_store = :redis_store, {expires_in: 1.month}
+```
+
+Default session id is made from bot's username and `(from || chat)['id']`.
+It means that session will be the same for updates from user in every chat,
+and different for every user in the same group chat.
+To change this behavior you can override `session_key` method, or even
+define multiple sessions in single controller. For details see `Session` module.
+
+```ruby
 class Telegram::WebhookController < Telegram::Bot::UpdatesController
   include Telegram::Bot::UpdatesController::Session
   # or just shortcut:
@@ -222,12 +243,16 @@ class Telegram::WebhookController < Telegram::Bot::UpdatesController
   end
 
   private
-  # By default it uses bot's username and user's id as a session key.
-  # Chat's id is used only when `from` field is empty.
-  # Override `session_key` method to change this behavior.
+
+  # In this case session will persist for user only in specific chat.
+  # Same user in other chat will have different session.
   def session_key
-    # In this case session will persist for user only in specific chat:
-    "#{bot.username}:#{chat['id']}:#{from['id']}"
+    "#{bot.username}:#{chat['id']}:#{from['id']}" if chat && from
+  end
+
+  # This session will be the same for all updates in chat.
+  def chat_session
+    @_chat_session ||= self.class.build_session(chat && "#{bot.username}:#{chat['id']}")
   end
 end
 ```

lib/telegram/bot/updates_controller/session.rb

@@ -8,19 +8,26 @@ module Telegram
       module Session
         extend ActiveSupport::Concern
 
+        module ClassMethods
+          # Builds session with given key and optional store (default to session_store).
+          # This way it's easier to define multiple custom sessions,
+          # ex. one for group chat and one for user.
+          def build_session(key, store = session_store)
+            raise 'session_store is not configured' unless store
+            key ? SessionHash.new(store, key) : NullSessionHash.new
+          end
+        end
+
         def process_action(*)
           super
         ensure
-          session.commit
+          session.commit if @_session
         end
 
         protected
 
         def session
-          @_session ||= begin
-            key = session_key
-            key ? SessionHash.new(self.class.session_store, key) : NullSessionHash.new
-          end
+          @_session ||= self.class.build_session(session_key)
         end
 
         def session_key

spec/telegram/bot/updates_controller/session_spec.rb

@@ -54,4 +54,45 @@ RSpec.describe Telegram::Bot::UpdatesController::Session do
       end
     end
   end
+
+  describe '.build_session' do
+    subject { controller_class.build_session(key, *args) }
+    let(:key) {}
+    let(:args) { [] }
+    it { expect { subject }.to raise_error(/session_store is not configured/) }
+
+    shared_examples 'NullSessionHash when key is not present' do |store_proc|
+      it { should be_instance_of(described_class::NullSessionHash) }
+
+      context 'and key is present' do
+        let(:key) { :test_key }
+        it 'is valid SessionHash' do
+          expect(subject).to be_instance_of(described_class::SessionHash)
+          expect(subject.id).to eq key
+          expect(subject.instance_variable_get(:@store)).to be(instance_exec(&store_proc))
+        end
+      end
+    end
+
+    context 'when store configured' do
+      before { controller_class.session_store = nil }
+      include_examples 'NullSessionHash when key is not present',
+        -> { controller_class.session_store }
+    end
+
+    context 'when store is given' do
+      let(:args) { [double(:store)] }
+      include_examples 'NullSessionHash when key is not present', -> { args[0] }
+    end
+  end
+
+  describe '.session_store=' do
+    subject { ->(val) { controller_class.session_store = val } }
+    it 'casts to AS::Cache' do
+      expect { subject[:null_store] }.to change(controller_class, :session_store).
+        to(instance_of(ActiveSupport::Cache::NullStore))
+      expect { subject[nil] }.to change(controller_class, :session_store).
+        to(instance_of(ActiveSupport::Cache::MemoryStore))
+    end
+  end
 end