cookbook article for https://gitlab.grifart.cz/ivy/server/-/merge_requests/222/diffs#note_69354

Zkusil jsem popsat, jak jsem slíbil.

docs/cookbook/migrate-to-composite-field.md

+# Migrate to composite field
+
+Sometimes you start with table containing values, which you then see have dependency on each other. As happened with [Stamp object](https://gitlab.grifart.cz/ivy/server/-/merge_requests/222/diffs#note_69354) (originally named `auditTrail`).
+
+## 1. migrating database
+
+```sql
+-- prepare new data type in PostgreSQL
+CREATE TYPE "public"."auditTrail" AS (
+	"occurredAt" timestamp without time zone,
+	"causedBy" uuid
+);
+
+-- Add column with default value
+ALTER TABLE "clinic"."patient"
+	ADD COLUMN "created" "public"."auditTrail" DEFAULT NULL;
+
+-- migrate data from two original columns to the new composite one
+UPDATE "clinic"."patient" SET
+	"created" = ROW("createdAt", "createdBy");
+
+-- removed old one and remove DEFAULT value (as it should be required filed at the end)
+ALTER TABLE "clinic"."patient"
+	ALTER COLUMN "created" SET NOT NULL,
+	ALTER COLUMN "created" DROP DEFAULT,
+	DROP COLUMN "createdAt",
+	DROP COLUMN "createdBy";
+```
+
+And add mapping of new field. It is useful to use PostgreSQL tools for composite & array types. This helper class will provide you logic for (de)serializing these composite types into standard SQL query.
+
+```php
+<?php
+		$phpToDatabaseMapper->addMapping(
+			static fn(string $dbType): ?string => $dbType === 'stamp' ? Stamp::class : null,
+			static fn(Stamp $trail): string => PostgreSQLTools::toPgComposite([
+				(string) $trail->getOccurredAt(),
+				$trail->getCausedBy()->toString(),
+			]),
+		);
+		$databaseToPhpMapper->addMapping(
+			static fn(string $dbType): ?string => $dbType === 'stamp' ? Stamp::class : null,
+			static function (string $value): Stamp {
+				[$occurredAt, $causedBy] = PostgreSQLTools::fromPgComposite($value);
+				$occurredAtLocal = LocalDateTime::parse($occurredAt, ISO8601Parsers::dateTime());
+				return Stamp::from(
+					AccountId::of($causedBy),
+					$occurredAtLocal->atTimeZone(TimeZone::utc())->getInstant(),
+				);
+			},
+		);
+```
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