Postgres function to show sum of user's credits

Desired state: We can query a graphql field userTotalCredits with an user ID, and get back a total sum of user's credits.

• get familiar with the "user" and "user_item_credit" tables in ark_data_model.dbml and ark_data_model.sql
• write a plpgsql postgres function that takes user_id as a parameter, does something like SELECT SUM(nb_credit) FROM user_item_credit WHERE user_id = 1 (but user_id is taken from the function parameter).

Useful documents:

• https://www.graphile.org/postgraphile/postgresql-schema-design/#database-functions
• https://github.com/graphile/starter/blob/main/data/schema.sql
• https://www.howtographql.com/