Fixing the Spring Boot Error: Character Varying and Smallint Types Do Not Have an Operator

Common Pitfalls with Spring Boot SQL Queries: Handling Type Mismatches in PostgreSQL

As developers, we've all encountered cryptic error messages that seem to come out of nowhere. One minute, our Spring Boot application is running smoothly; the next, we’re staring at an error about incompatible data types. 😅 It’s both frustrating and perplexing, especially when dealing with complex query setups.

Recently, I ran into a PostgreSQL error in Spring Boot: "operator does not exist: character varying = smallint." This message showed up while attempting to use a Set of enums in an SQL query's IN clause. The mismatch between the enum type and the database column type created an unexpected hiccup in what seemed like straightforward code.

While it's tempting to blame database quirks or Spring Boot, the real issue often lies in how enums and database types are mapped. Java enums, when mapped to databases, require special handling, especially with PostgreSQL. Understanding these details can save time and prevent future issues when working with enums in Spring Boot.

In this guide, I'll explain how I identified the problem and walked through a practical solution. From my own debugging journey to specific code fixes, you'll gain the tools you need to avoid type mismatches in your queries and ensure seamless database interactions. 🔧

Resolving Data Type Mismatches in Spring Boot with PostgreSQL

When working with Spring Boot and PostgreSQL, developers often encounter type mismatch issues, especially with enums. In this case, the error "operator does not exist: character varying = smallint" happens because PostgreSQL can’t directly interpret a Java enum as a SQL type in native queries. Here, the SystemAccounts entity includes a userCode field represented by the AccountType enum, which maps values like "PERSONAL" or "CORPORATE" in Java. However, when attempting a native SQL query with a Set of enums, PostgreSQL can't automatically match enum types, resulting in this error. To overcome this, it's crucial to convert the enum to a string before passing it to the query. 🎯

In the solution provided, we start by adjusting the enum mapping in SystemAccounts using the @Enumerated(EnumType.STRING) annotation. This instructs JPA to store each AccountType as a readable string instead of a numeric ordinal. It’s a small change, but it simplifies future data handling by avoiding numerical values, which would make debugging complex in the database. In our repository, we can then use a custom @Query annotation to specify the SQL logic. However, since PostgreSQL still needs enums as strings in the query, we need to process the AccountType values into a string format before passing them in.

The CriteriaBuilder API offers a dynamic solution to this issue. Using CriteriaBuilder, we can avoid native SQL by building queries programmatically in Java. This approach enables us to add enum filters without writing SQL by hand, which reduces SQL errors and helps with maintainability. In our script, we create a list of Predicate conditions based on each enum’s string value, using cb.equal() to match each AccountType in the Set. Then, cb.or() combines these predicates, allowing multiple values in the same query. This flexible setup dynamically manages the enum-to-string conversion, minimizing compatibility issues with PostgreSQL.

Finally, the solution incorporates a unit test to verify compatibility. Using JUnit, we confirm that each AccountType works with our query, validating that the userCode field can store "PERSONAL" or "CORPORATE" values and retrieve them without errors. This test method first sets up the required AccountType values and runs the findAllByUserCodes() query to check results. Adding assertNotNull() and assertTrue() checks guarantees we don’t encounter null or incorrect values, ensuring our solution handles all cases effectively. With this setup, the application is better prepared to handle enum queries across various conditions in production. 🧪

// Problem: PostgreSQL expects specific data types in queries.
// Solution: Convert enums to strings for query compatibility with PostgreSQL.
// This Spring Boot backend solution is clear, optimized, and includes type checks.

@Entity
@Table(name = "system_accounts")
@Getter
@Setter
public class SystemAccounts {
    @Id
    @Column(name = "id", nullable = false)
    private UUID id;
    @Column(name = "user_code")
    private String userCode;  // Store as String to avoid type mismatch
}

// Enumeration for AccountType
public enum AccountType {
    PERSONAL,
    CORPORATE
}

// Repository Query with Enum Handling
@Query(value = """
    SELECT sa.id FROM system_accounts sa
    WHERE sa.user_code IN :accountTypes""", nativeQuery = true)
Optional<List<SystemAccounts>> findAllByUserCodes(@Param("accountTypes") List<String> accountTypes);
// This query accepts a List of strings to avoid casting issues.

// Convert AccountType enums to Strings in Service
List<String> accountTypeStrings = accountTypes.stream()
    .map(Enum::name)
    .collect(Collectors.toList());

// Using CriteriaBuilder to dynamically handle enums in queries with automatic type checking.
// This approach uses Java’s Criteria API to avoid type mismatches directly in the code.

public List<SystemAccounts> findAllByUserCodes(Set<AccountType> accountTypes) {
    CriteriaBuilder cb = entityManager.getCriteriaBuilder();
    CriteriaQuery<SystemAccounts> query = cb.createQuery(SystemAccounts.class);
    Root<SystemAccounts> root = query.from(SystemAccounts.class);
    Path<String> userCodePath = root.get("userCode");
    List<Predicate> predicates = new ArrayList<>();

    // Add predicates for enum values, converting to String for matching
    for (AccountType type : accountTypes) {
        predicates.add(cb.equal(userCodePath, type.name()));
    }

    query.select(root)
         .where(cb.or(predicates.toArray(new Predicate[0])));
    return entityManager.createQuery(query).getResultList();
}

// This JUnit test ensures both solutions handle enums correctly with PostgreSQL.
// Tests database retrieval for both AccountType values: PERSONAL and CORPORATE.

@SpringBootTest
public class SystemAccountsRepositoryTest {
    @Autowired
    private SystemAccountsRepository repository;

    @Test
    public void testFindAllByUserCodes() {
        Set<AccountType> accountTypes = Set.of(AccountType.PERSONAL, AccountType.CORPORATE);
        List<SystemAccounts> results = repository.findAllByUserCodes(accountTypes);

        // Verify results are returned and types match
        assertNotNull(results);
        assertTrue(results.size() > 0);
        results.forEach(account ->
            assertTrue(account.getUserCode().equals("PERSONAL") || account.getUserCode().equals("CORPORATE"))
        );
    }
}

Handling Enum to String Conversion in PostgreSQL with Spring Boot

When using Spring Boot with PostgreSQL, handling enums in database queries often requires special attention, particularly when enums are involved in native SQL queries. By default, PostgreSQL doesn't support Java enums directly, and instead expects a compatible data type like varchar or text in queries. For instance, when we need to filter results based on an enum like AccountType, PostgreSQL requires us to convert the Java enum to a string value before executing the query. This conversion prevents the common “operator does not exist” error, which occurs when the database attempts to compare an enum with an incompatible type like smallint or character varying.

One way to handle this issue is to leverage the @Enumerated(EnumType.STRING) annotation in Spring Boot, which stores enums as string values directly in the database. However, for scenarios involving native queries, it’s often necessary to convert the enums into strings within the query itself. By using methods like .stream() and map(Enum::name), we can generate a list of string representations for our enum values, which can then be passed to PostgreSQL without any type mismatch issues. This approach ensures flexibility, allowing us to filter by multiple AccountType values seamlessly without errors.

For applications with more complex enum usage, another approach is to use the CriteriaBuilder API, which lets us dynamically construct queries in a type-safe way without manually writing SQL. This API is particularly useful for creating reusable and database-agnostic code, as it translates enums into compatible database types automatically, reducing the risk of type errors. With this method, the query construction process is simplified, and developers gain the flexibility to handle various enum-based filters in a unified way.