Recover from three kinds of init error

- discovering migrations pending on dfe-analytics init
- discovering missing database connection when database not present
- loading models before connecting to DB

The final example is important because some environments which lack
db connections (eg asset precompilation) also don't provide all
necessary eg ENV vars which would be required were we loading the whole
app (which this init process does).

So discover early if we have no database (call
ActiveRecord::Base.connection). If anything happens during Fields.check!
catch that too). Log lines will tell us if we're bailing for want of a
DB or for want of a migration. (There's no point in running check! if
migration yet to run).

lib/dfe/analytics.rb

@@ -79,13 +79,7 @@ def self.configure
     end
 
     def self.initialize!
-      begin
-        ActiveRecord::Base.connection
-      rescue ActiveRecord::ActiveRecordError
-        Rails.logger.info('No database connection; DfE Analytics not initialized')
-        return
-      end
-
+      ActiveRecord::Base.connection # cause an exception early if we can't connect
       DfE::Analytics::Fields.check!
 
       entities_for_analytics.each do |entity|
@@ -96,6 +90,10 @@ def self.initialize!
           model.include(DfE::Analytics::Entities)
         end
       end
+    rescue ActiveRecord::PendingMigrationError
+      Rails.logger.info('Database requires migration; DfE Analytics not initialized')
+    rescue ActiveRecord::ActiveRecordError
+      Rails.logger.info('No database connection; DfE Analytics not initialized')
     end
 
     def self.enabled?
@@ -177,9 +175,12 @@ def self.entity_model_mapping
 
         Rails.application.eager_load!
 
+        rails_tables = %w[ar_internal_metadata schema_migrations]
+
         ActiveRecord::Base.descendants
-         .reject(&:abstract_class?)
-         .index_by(&:table_name)
+          .reject(&:abstract_class?)
+          .index_by(&:table_name)
+          .except(*rails_tables)
       end
     end
 

spec/dfe/analytics_spec.rb

@@ -22,6 +22,15 @@
     end
   end
 
+  describe 'when migrations are pending' do
+    it 'recovers and logs' do
+      allow(DfE::Analytics::Fields).to receive(:check!).and_raise(ActiveRecord::PendingMigrationError)
+
+      allow(Rails.logger).to receive(:info).with(/Database requires migration/)
+      expect { DfE::Analytics.initialize! }.not_to raise_error
+    end
+  end
+
   describe 'field checks on initialization' do
     # field validity is computed from allowlist, blocklist and database. See
     # Analytics::Fields for more details