Fix compatibility with older active record versions

Author: Igor Špánik

lib/standby/connection_holder.rb

@@ -6,7 +6,13 @@ class << self
       # for delayed activation
       def activate(target)
         env_name = "#{ActiveRecord::ConnectionHandling::RAILS_ENV.call}_#{target}"
-        spec = ActiveRecord::Base.configurations.find_db_config(env_name)&.configuration_hash
+
+        if ActiveRecord.version >= Gem::Version.new('6.0')
+          spec = ActiveRecord::Base.configurations.find_db_config(env_name).try(:configuration_hash)
+        else
+          spec = ActiveRecord::Base.configurations[env_name]
+        end
+
         raise Error, "Standby target '#{target}' is invalid!" if spec.nil?
 
         establish_connection spec

spec/configuration_spec.rb

@@ -4,13 +4,22 @@
   before do
     # Backup connection and configs
     @backup_conn = Standby.instance_variable_get :@standby_connections
-    @backup_config = ActiveRecord::Base.configurations.configs_for.map do |config|
-      [config.env_name, config.configuration_hash]
-    end.to_h
     @backup_disabled = Standby.disabled
+
+    if ActiveRecord.version >= Gem::Version.new('6.0')
+      @backup_config = ActiveRecord::Base.configurations.configs_for.map do |config|
+        [config.env_name, config.configuration_hash]
+      end.to_h
+      @empty_config = {}
+    else
+      @backup_config = ActiveRecord::Base.configurations.dup
+      @empty_config = nil
+    end
+
     @backup_conn.each_key do |klass_name|
       Object.send(:remove_const, klass_name) if Object.const_defined?(klass_name)
     end
+
     Standby.instance_variable_set :@standby_connections, {}
   end
 
@@ -22,13 +31,13 @@
   end
 
   it 'raises error if standby configuration not specified' do
-    ActiveRecord::Base.configurations = @backup_config.merge({ 'test_standby' => {} })
+    ActiveRecord::Base.configurations = @backup_config.merge({ 'test_standby' => @empty_config })
 
     expect { Standby.on_standby { User.count } }.to raise_error(Standby::Error)
   end
 
   it 'connects to primary if standby configuration is disabled' do
-    ActiveRecord::Base.configurations = @backup_config.merge({ 'test_standby' => {} })
+    ActiveRecord::Base.configurations = @backup_config.merge({ 'test_standby' => @empty_config })
     Standby.disabled = true
 
     expect(Standby.on_standby { User.count }).to be 2