test: add postgres migration tests

src/test/java/com/slotline/migration/PostgresMigrationTest.java

@@ -0,0 +1,96 @@
+package com.slotline.migration;
+
+import org.flywaydb.core.Flyway;
+import org.junit.jupiter.api.Assumptions;
+import org.junit.jupiter.api.Test;
+import org.testcontainers.DockerClientFactory;
+import org.testcontainers.containers.PostgreSQLContainer;
+
+import java.sql.Connection;
+import java.sql.DriverManager;
+import java.sql.ResultSet;
+import java.sql.SQLException;
+import java.sql.Statement;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+class PostgresMigrationTest {
+
+    @Test
+    void migrationsApplyAndPartialUniqueIndexKeepsOneActiveSlot() throws Exception {
+        Assumptions.assumeTrue(DockerClientFactory.instance().isDockerAvailable(),
+                "Docker is not available; skipping the PostgreSQL migration test");
+
+        try (PostgreSQLContainer<?> postgres = new PostgreSQLContainer<>("postgres:16-alpine")) {
+            postgres.start();
+
+            Flyway flyway = Flyway.configure()
+                    .dataSource(postgres.getJdbcUrl(), postgres.getUsername(), postgres.getPassword())
+                    .locations("classpath:db/migration/postgresql")
+                    .load();
+            flyway.migrate();
+
+            try (Connection connection = DriverManager.getConnection(
+                    postgres.getJdbcUrl(), postgres.getUsername(), postgres.getPassword())) {
+
+                long organizationId = single(connection, "SELECT id FROM organization WHERE slug = 'default'");
+                long userId = returningId(connection, "INSERT INTO app_user "
+                        + "(organization_id, username, password_hash, role, created_at) "
+                        + "VALUES (" + organizationId + ", 'pg-user', 'x', 'USER', now()) RETURNING id");
+                long resourceId = returningId(connection, "INSERT INTO resource "
+                        + "(organization_id, name, description, zone_id, buffer_minutes, created_at, version) "
+                        + "VALUES (" + organizationId + ", 'PG court', 'outdoor', 'UTC', 0, now(), 0) RETURNING id");
+
+                long firstReservationId = returningId(connection, activeReservationInsert(resourceId, organizationId,
+                        userId));
+
+                boolean duplicateRejected = false;
+                try (Statement statement = connection.createStatement()) {
+                    statement.execute(activeReservationInsert(resourceId, organizationId, userId));
+                } catch (SQLException uniqueViolation) {
+                    duplicateRejected = true;
+                }
+                assertThat(duplicateRejected)
+                        .as("partial unique index must reject a second ACTIVE reservation on the same slot")
+                        .isTrue();
+
+                try (Statement statement = connection.createStatement()) {
+                    statement.executeUpdate(
+                            "UPDATE reservation SET status = 'CANCELLED' WHERE id = " + firstReservationId);
+                }
+
+                boolean rebookAllowed = true;
+                try (Statement statement = connection.createStatement()) {
+                    statement.execute(activeReservationInsert(resourceId, organizationId, userId));
+                } catch (SQLException stillBlocked) {
+                    rebookAllowed = false;
+                }
+                assertThat(rebookAllowed)
+                        .as("once the slot is cancelled the same slot may be booked again")
+                        .isTrue();
+            }
+        }
+    }
+
+    private String activeReservationInsert(long resourceId, long organizationId, long userId) {
+        return "INSERT INTO reservation "
+                + "(resource_id, organization_id, user_id, start_time, end_time, status, created_at, version) "
+                + "VALUES (" + resourceId + ", " + organizationId + ", " + userId + ", "
+                + "TIMESTAMPTZ '2026-09-01 10:00:00+00', TIMESTAMPTZ '2026-09-01 11:00:00+00', "
+                + "'ACTIVE', now(), 0)";
+    }
+
+    private long single(Connection connection, String sql) throws SQLException {
+        try (Statement statement = connection.createStatement(); ResultSet rows = statement.executeQuery(sql)) {
+            rows.next();
+            return rows.getLong(1);
+        }
+    }
+
+    private long returningId(Connection connection, String sql) throws SQLException {
+        try (Statement statement = connection.createStatement(); ResultSet rows = statement.executeQuery(sql)) {
+            rows.next();
+            return rows.getLong(1);
+        }
+    }
+}