DBEvents.md

DB Events

• DB Events are functions/callbacks to be executed on responding to some action like Model update/delete/create
• More info on this topic and list of all supported [SQLALCHEMY Events](- List of all SqlAlchemy events)

Registering a new hook/event

• All events are added in package crm.events

• Add a new module there with a function call back to be called when some event happen

• Register your function call back by binding it to some event

• Example: crm.events.catch_db_updates.py

  from sqlalchemy.event import listen
  
  from crm.db import db
  
  
  def catch_db_updates_after_flush(db_session, flush_context):
  
      db_session.info['changes'] = {
          'updated': db_session.dirty,
          'created': db_session.new,
          'deleted': db_session.deleted
      }
  
  listen(db.session, 'after_flush', catch_db_updates_after_flush)

  • db.session.new contains all newly created objects but, No many to many models are included
  • db.session.dirty contains all updated objects
  • db.session.deleted contains list of deleted objects

WARNING

• DB Events are disabled when you execute command flask loaddata because usually you don't want some actions that may manipulate your data to be executed when you try to load a data mirror in physical DB

• Most Events won't work with bulk updates and bulk inserts unfortunately

Explanation of existing DB events handlers in CRM

crm.events.update_auto_fields.py

• registers an after_flush event callback which gets newly created non many to many models and assign them a unique ID string and add original author id to model. Also add latest author user id to model

crm.events.catch_db_updates.py

• registers an after_flush event callback which gets values of db.session.new, db.session.dirty and db.session.deleted and cache them in db.session.info['changes'] because we need to access these data when the event after_transaction is triggered but during after_transaction we have a problem that db.session.new, db.session.dirty and db.session.deleted are all empty, so we cache these data in temp location where we can access them in the appropriate time

crm.events.cache_db_updates.py

• registers an after_transaction event callback which gets all db updates saved in db.session.info['changes'] and adds them to the existing cache backend. so we can keep track of any change to DB in memory and we can dump it into JSON files in DATA_DIR automatically