Default columns should generate values from postgres

If you're using something like Hasura to connect through Lucky, the GraphQL may need to insert records in to postgres at some point. If you happen to have Avram setup with UUID, then your ID, and the timestamps are all generated from Crystal. This means that inserting a record through GraphQL fails.

To temporarily fix this, you can create a migration that runs this:

def migrate
  enable_extension "uuid-ossp" # NOTE: this requires Avram > 0.15? (as of today, it isn't released yet)
  execute("ALTER TABLE users ALTER COLUMN id SET DEFAULT uuid_generate_v4();")
  execute("ALTER TABLE users ALTER COLUMN created_at SET DEFAULT CURRENT_TIMESTAMP;")
  execute("ALTER TABLE users ALTER COLUMN updated_at SET DEFAULT CURRENT_TIMESTAMP;")
end

/cc. @russ @KCErb

Start with src/avram/save_operation.cr around lines 327-328 and review how Avram currently generates UUID and timestamp defaults. Compare that behavior with the migration workaround in the issue; done means records inserted through GraphQL can rely on PostgreSQL-generated id, created_at, and updated_at values.