sql alchemy column value dependent on other table - sqlalchemy

Question

If there's a table with a column that I want to get the number of occurrences of the columns 'id' in another tables column?
So if there was a table 'player' of every player, and a table 'goals' that listed every goal scored, is there an easy way to autoupdate the player column every time a goal they score is added to the goal table?
another example would be a 'team' and 'players' table, where the table updates the team.number_of_players every time a player is added with player.team_name == team.name or something like that.
Would using JSON as a way of holding {'username': True} or something like that for each user be worthwhile?

Answer 1

You have several ways to implement you idea:
Easiest way: you can update your columns with update query, something like this:
try:
player = Player(name='New_player_name', team_id=3)
Session.add(player)
Session.flush()
Session.query(Team).filter(Team.id == Player.team_id).update({Team.players_number: Team.players_number + 1})
Session.commit()
except SQLAlchemyError:
Session.rollback()
# error processing
You can implement sql-trigger. But an implementation is different for different DBMS. So, you can read about it in the documentation of your DBMS.
You can implement SQLAlchemy trigger, like this:
from sqlalchemy import event
class Team(Base):
...
class Player(Base):
...
#staticmethod
def increment_players_number(mapper, connection, player):
try:
Session.query(Team).filter(Team.id == player.team_id)\
.update({Team.players_number: Team.players_number + 1})
except SQLAlchemyError:
Session.rollback()
# error processing
event.listen(Player, 'after_insert', Player.increment_players_number)
As you see, there are always two queries, because you should perform two procedures: insert and update. I think (but I'm not sure) that some DBMS can process queries like this:
UPDATE table1 SET column = column + 1 WHERE id = SOMEID AND (INSERT INTO table2 values (VALUES))