santoshD13
I have 2 tables trainings and training_instances. Please see the schema below:
trainings table:
column_name data_type is_nullable. foreign_key
id uuid NO NULL
day int4 NO NULL
start_time time(0) NO NULL
end_time time(0) NO NULL
team_id uuid YES public.teams(id)
inserted_at timestamp(0) NO NULL
updated_at timestamp(0) NO NULL
Note that trainings table does not have a date or start_date column yet. The column named day is the day of the week. One training_id can have multiple training instances.
training_instances table:
column_name data_type is_nullable. foreign_key
id uuid NO NULL
date date NO NULL
start_time time(0) NO NULL
end_time time(0) NO NULL
team_size int4 YES NULL
training_id uuid YES public.trainings(id)
team_id uuid YES public.teams(id)
inserted_at timestamp(0) NO NULL
updated_at timestamp(0) NO NULL
I am trying to create a new column named start_date in the trainings table and then insert the data here for existing trainings. I want to insert for every training the oldest date for its corresponding training instance in this new start_date column.
What would be the best way to write a migration here?
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03juan
Welcome to the forum
I would separate the schema migration and data updating into separate concerns. See José’s answer https://stackoverflow.com/a/48382065.
And generally I wouldn’t do the data update in a migration script because if you import your schema into the migration it’ll work at the time that you’re doing this execise, but if you re-run your migrations to setup a new database in the future, there is no guarantee that the Elixir schema hasn’t been updated in the meantime. For example, if in the future you do another structural change, the future Elixir schema may not match the shape of what the older migration is trying to do and it’ll just crash.
Rather put the temporary data migration in a Mix.Task that you run once and then can delete from your code’s repo when you’re done, to prevent hitting a wall in the future.