Is there a way to create one JPA entity based on many database tables and do I really have to do this or is it a bad practice?

Question

I'm quite new to Spring Data JPA technology and currently facing one task I can't deal with. I am seeking best practice for such cases.

In my Postgres database I have a two tables connected with one-to-many relation. Table 'account' has a field 'type_id' which is foreign key references to field 'id' of table 'account_type':

So the 'account_type' table only plays a role of dictionary. Accordingly to that I've created to JPA entities (Kotlin code):

@Entity
class Account(
  @Id @GeneratedValue var id: Long? = null,
  var amount: Int,
  @ManyToOne var accountType: AccountType
)
@Entity
class AccountType(
  @Id @GeneratedValue var id: Long? = null,
  var type: String
)

In my Spring Boot application I'd like to have a RestConroller which will be responsible for giving all accounts in JSON format. To do that I made entities classes serializable and wrote a simple restcontroller:

@GetMapping("/getAllAccounts", produces = [APPLICATION_JSON_VALUE])
fun getAccountsData(): String {
    val accountsList = accountRepository.findAll().toMutableList()
    return json.stringify(Account.serializer().list, accountsList)
}

where accountRepository is just an interface which extends CrudRepository<Account, Long>.

And now if I go to :8080/getAllAccounts, I'll get the Json of the following format (sorry for formatting):

[
  {"id":1,
   "amount":0,
   "accountType":{
      "id":1,
      "type":"DBT"
     }
  },
  {"id":2,
    "amount":0,
    "accountType":{
       "id":2,
       "type":"CRD"
      }
   }
]

But what I really want from that controller is just

[
   {"id":1,
    "amount":0,
    "type":"DBT"
   },
   {"id":2,
    "amount":0,
    "type":"CRD"
   }
]

Of course I can create new serializable class for accounts which will have String field instead of AccountType field and can map JPA Account class to that class extracting account type string from AccountType field. But for me it looks like unnecessary overhead and I believe that there could be a better pattern for such cases.

For example what I have in my head is that probably somehow I can create one JPA entity class (with String field representing account type) which will be based on two database tables and unnecessary complexity of having inner object will be reduced automagically each time I call repository methods :) Moreover I will be able to use this entity class in my business logic without any additional 'wrappers'.

P.s. I read about @SecondaryTable annotation but it looks like it can only work in cases where there is one-to-one relation between two tables which is not my case.

Answer 1

There are a couple of options whic allow clean separation without a DTO.

Firstly, you could look at using a projection which is kind of like a DTO mentioned in other answers but without many of the drawbacks:

https://docs.spring.io/spring-data/jpa/docs/current/reference/html/#projections

@Projection(
  name = "accountSummary", 
  types = { Account.class }) 
public Interface AccountSummaryProjection{

    Long getId();

    Integer getAmount();

    @Value("#{target.accountType.type}")
    String getType();
}

You then simply need to update your controller to call either query method with a List return type or write a method which takes a the proection class as an arg.

https://docs.spring.io/spring-data/jpa/docs/current/reference/html/#projection.dynamic

@GetMapping("/getAllAccounts", produces = [APPLICATION_JSON_VALUE])
@ResponseBody
fun getAccountsData(): List<AccountSummaryProjection>{
    return accountRepository.findAllAsSummary();
}

An alternative approach is to use the Jackson annotations. I note in your question you are manually tranforming the result to a JSON String and returning a String from your controller. You don't need to do that if the Jackson Json library is on the classpath. See my controller above.

So if you leave the serialization to Jackson you can separate the view from the entity using a couple of annotations. Note that I would apply these using a Jackson mixin rather than having to pollute the Entity model with Json processing instructions however you can look that up:

@Entity
class Account(

  //in real life I would apply these using a Jacksin mix
  //to prevent polluting the domain model with view concerns.
  @JsonDeserializer(converter = StringToAccountTypeConverter.class)
  @JsonSerializer(converter = AccountTypeToStringConverter.class
  @Id @GeneratedValue var id: Long? = null,
  var amount: Int,
  @ManyToOne var accountType: AccountType
)

You then simply create the necessary converters:

public class StringToAccountTypeConverter extends StdConverter<String, CountryType> 
           implements org.springframework.core.convert.converter.Converter<String, AccountType> {

  @Autowired
  private AccountTypeRepository repo;

  @Override
  public AccountType convert(String value) {
      //look up in repo and return
  }
}

and vice versa:

public class AccountTypeToStringConverter extends StdConverter<String, CountryType> 
           implements org.springframework.core.convert.converter.Converter<AccountType, String> {

  @Override
  public String convert(AccountType value) {
      return value.getName();
  }
}

Answer 2

One of the least complicated ways to achieve what you are aiming for - from the external clients' point of view, at least - has to do with custom serialisation, what you seem to be aware of and what @YoManTaMero has extended upon.

Obtaining the desired class structure might not be possible. The closest I've managed to find is related to the @SecondaryTable annotation but the caveat is this only works for @OneToOne relationships.

In general, I'd pinpoint your problem to the issue of DTOs and Entities. The idea behind JPA is to map the schema and content of your database to code in an accessible but accurate way. It takes away the heavy-lifting of managing SQL queries, but it is designed mostly to reflect your DB's structure, not to map it to a different set of domains.

If the organisation of your DB schema does not exactly match the needs of your system's I/O communication, this might be a sign that:

• Your DB has not been designed correctly;
• Your DB is fine, but the manageable entities (tables) in it simply do not match directly to the business entities (models) in your external communication.

Should second be the case, Entities should be mapped to DTOs which can then be passed around. Single Entity may map to a few different DTOs. Single DTO might take more than one (related!) entities to be created. This is a good practice for medium-to-large systems in the first place - handing out references to the object that's the direct access point to your database is a risk.

Mind that simply because the id of the accountType is not taking part in your external communication does not mean it will never be a part of your business logic.

To sum up: JPA is designed with ease of database access in mind, not for smoothing out external communication. For that, other tools - such as e.g. Jackson serializer - are used, or certain design patterns - like DTO - are being employed.

Answer 3

One approach to solve this is to @JsonIgnore accountType and create getType method like

@JsonProperty("type")   
var getType() {
    return accountType.getType();
}