Many to one relation beetween 3 entities

Question

In my springboot application I have these 3 entities:

@Entity
public class Process {
    @Id
    private Long Id;

    @ManyToOne(fetch = FetchType.EAGER, cascade = {CascadeType.ALL})
    @JoinColumn(name = "input_id")
    private Input input;
    ...
}

@Entity
public class Input{
    @Id
    private Long Id;

    @ManyToOne(fetch = FetchType.EAGER)
    @JoinColumn(name = "template_id")
    private Template template;
    ...
}

@Entity
public class Template{
    @Id
    private Long Id;

    @ManyToOne(fetch = FetchType.EAGER)
    @JoinColumn(name = "template_id")
    private Template template;
    
    private String name;    
    ...
}

In summary, Process has an FK to Input and Input has an FK to Template.

I would like to filter the processes whose template have a certain name. Here is the SQL I would to perform something like that:

    select
        *
    from
        process p 
    left outer join
        input i 
            on p.input_id=i.id 
    left outer join
        template t 
            on i.template_id=t.id 
    where
        t.name='templateName'

Here is what I currently have in my Process entity to access the template:

    @ManyToOne
    @JoinTable(name = "Input",
            joinColumns = {@JoinColumn(table = "Input", referencedColumnName = "id")},
            inverseJoinColumns = {
                    @JoinColumn(name = "template_id", referencedColumnName = "id", table = "Template")})
    private Template template;

Here is my ProcessRepository class, where I now have access to the desired find method:

@Repository
public interface ProcessRepository extends PagingAndSortingRepository<Process, Long> {
    ...
    List<Process> findByTemplateNameEquals(String templateName);
    ...
}

When I execute the findByTemplateNameEquals method, I retrieve the process and one template. But the result I got was not the one expected. I enabled the sql logging and here is the query really performed (I hide the columns, it is not important here):

    select
...
    from
        process process0_ 
    left outer join
        input process0_1_ 
            on process0_.id=process0_1_.id 
    left outer join
        template template1_ 
            on process0_1_.template_id=template1_.id 
    where
        template1_.name=?

There is one problem with the join between Process and Input. It executes

from
  process process0_ 
left outer join
  input process0_1_ 
  on process0_.id=process0_1_.id

instead of

from
  process process0_ 
left outer join
  input process0_1_ 
  on process0_.input_id=process0_1_.id

I don't understand why it use the PK of Process instead of the FK to Input. I tried several things to solve this:

1. Adding name="input_id" in the joinColumns = {@JoinColumn(... but instead of replacing the FK, it replaces the PK of input => failure during execution
2. replacing the referencedColumnName by "input_id" in the joinColumns = {@JoinColumn(... but it failed at launching.
3. Configuring a @ForeignKey(name = "input_id") at several places (directly in the @JoinTable, in the @JoinColumn and even in the @JoinColumn of the Input input attribute ) but there was no change.

I also remarked that the joinColumns = {@JoinColumn(table = "Input", referencedColumnName = "id")} was not necessary, because I have the same behaviour if I remove it.

Could someone help me on this?

Many thanks in advance

Answer 1

I think the declaration of the template field at the Process level is probably unnecessary because you already have the relationship with Input , and certainly error prone.

In any case, if necessary, I would define the field with something like that instead:

@ManyToOne
@JoinTable(
  name = "Input",
  joinColumns = { @JoinColumn(name = "input_id")},
  inverseJoinColumns = {@JoinColumn(name = "template_id")}
)
private Template template;

Pease, verify the code.

Having said that, if you only need to obtain the repositories associated with a certain template by name you can try to navigate through the object hierarchy in your find method. Please, try:

@Repository
public interface ProcessRepository extends PagingAndSortingRepository<Process, Long> {
    ...
    List<Process> findByInput_Template_Name(final String templateName);
    ...
}