Migration: enable domain changes (upcoming feature) (#2802)

* Migration: enable domain changes (upcoming feature)

* Pass domain check within the same site

priv/repo/migrations/20230328062644_allow_domain_change.exs

@@ -0,0 +1,49 @@
+defmodule Plausible.Repo.Migrations.AllowDomainChange do
+  use Ecto.Migration
+
+  def up do
+    alter table(:sites) do
+      add(:domain_changed_from, :string, null: true)
+      add(:domain_changed_at, :naive_datetime, null: true)
+    end
+
+    create(unique_index(:sites, :domain_changed_from))
+    create(index(:sites, :domain_changed_at))
+
+    execute("""
+    CREATE OR REPLACE FUNCTION check_domain() RETURNS TRIGGER AS $$
+    BEGIN
+      IF EXISTS (
+        SELECT 1 FROM sites
+         WHERE (NEW.domain = domain_changed_from AND NEW.id != id)
+         OR (OLD IS NULL AND NEW.domain_changed_from = domain)
+      ) THEN
+        RAISE unique_violation USING CONSTRAINT = 'domain_change_disallowed';
+      END IF;
+      RETURN NEW;
+    END;
+    $$ LANGUAGE plpgsql;
+    """)
+
+    execute("""
+    CREATE TRIGGER check_domain_trigger
+    BEFORE INSERT OR UPDATE ON sites
+    FOR EACH ROW EXECUTE FUNCTION check_domain();
+    """)
+  end
+
+  def down do
+    execute("""
+    DROP TRIGGER check_domain_trigger ON sites
+    """)
+
+    execute("""
+    DROP FUNCTION check_domain()
+    """)
+
+    alter table(:sites) do
+      remove(:domain_changed_from)
+      remove(:domain_changed_at)
+    end
+  end
+end