Joins parent and child tables in rails

1. Get parent objects:

Alert.joins("LEFT JOIN alert_events on alert_events.alert_id = alerts.id 

").group('alerts.id').select('alerts.*')

2. Get Child Object:

AlertEvent.joins("INNER JOIN alerts on alerts.id = alert_events.alert_id 
").group('alert_events.id').select('alert_events.*')

Active record for child object
AlertEvent.joins(:alert).where(alerts: {category: 'Pay'})

3. Joins multiple tables:

 @previous_seafood_items = SeafoodItem.joins("LEFT JOIN auction_items on auction_items.itemable_id = seafood_items.id and auction_items.itemable_type = 'SeafoodItem' and auction_items.auction_id is NULL")
    .group('seafood_items.id')
    .select('seafood_items.*')
    .where(user_id: current_user.id )

    .paginate(:page => params[:page], :per_page => 20)