Add Relation#on_conflict with a DSL for upserts

[flash-gordon]

Upserts required dropping down to Sequel with dataset.insert_conflict(target: ..., update: { name: Sequel[:excluded][:name] }), and Postgres::Upsert covers single inserts configured at the class level only. This makes ON CONFLICT a part of the relation, so it composes the way where does and any insert through such a relation becomes an upsert:

users.on_conflict(:email).insert(tuple)                        # ON CONFLICT (email) DO NOTHING
users.on_conflict(constraint: :users_email_key).insert(tuple)  # ON CONFLICT ON CONSTRAINT users_email_key DO NOTHING
users.on_conflict(:email) { active.is(true) }.insert(tuple)    # ON CONFLICT (email) WHERE active IS TRUE DO NOTHING

users.on_conflict(:email).do_update.multi_insert(tuples)       # DO UPDATE SET every column but the key and the target from EXCLUDED
users.on_conflict(:email).do_update(:name, :updated_at).multi_insert(tuples)
users.on_conflict(:email).do_update(removed_at: nil).insert(tuple)

do_update accepts a block evaluated with a DSL: attributes refer to the existing row, excluded to the row proposed for insertion, functions are resolved via the virtual row like in where. set and where return immutable values and chain:

user_presence
  .on_conflict(:user_uuid)
  .do_update {
    set(last_seen_at: greatest(last_seen_at, excluded[:last_seen_at]), updated_at: excluded[:updated_at])
      .where(updated_at < excluded[:updated_at])
  }
  .multi_insert(tuples)

Commands::Create is restrictable now, hence the relation flows into commands and changesets, which return the upserted rows via RETURNING on PostgreSQL:

users.on_conflict(:email).do_update(:name).command(:create).call(tuple)
users.on_conflict(:email).changeset(:create, data).commit

Works with PostgreSQL and SQLite, MySQL raises UnsupportedFeatureError. Relation#upsert and Postgres::Upsert are left untouched.

[flash-gordon]

It's an AI doing, I'll review it soon. I just accumulated a dozen upserts in the app and I've decided it's enough to model a DSL worth having (and matching our style!).