Discriminator columns and subtypes

[Marcel Overdijk]

Hi,

Does SimpleFlatMapper support mapping to specific types using a discriminator column?

E.g. I have an abstract Employee class with 2 subclasses: InternalEmployee and ExternalEmployee.

In the database this is stored in a single table having a type column containing the value "I" or "E". It's a legacy db model that cannot be changed.

Wen querying a list of Employees is it possible to map to the correct subclass using SimpleFlatMapper?

Cheers,

Marcel

[Arnaud Roger]

There is a Discriminator mapper thingy, that as far as I can recall would work only at the root type - it is quite a pain to do with the interaction with the joins.

you can look at those unit tests for some examples

https://github.com/arnaudroger/SimpleFlatMapper/blob/c33e018dcc80bf2f3409e6e5f7a86c531ded35f7/sfm-jdbc/src/test/java/org/simpleflatmapper/jdbc/test/DiscriminatorJdbcMapperTest.java

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[Marcel Overdijk]

Thanks,

I got the basics working.

Based on the .<Employee>newDiscriminator("type") and the .when definitions the correct subclassed is used to map the record to.

The thing I don't get to work yet is that the InternalEmployee class has a salary property, but the database column is named "internal_salary".

All columns name in the employee that are for specific type are prefixed with type. Just like ExternalEmployee has an hourRate property and the employee table contains an external_hour_rate column.

Is this something that can be mapped?

In the link with the test case I see .addMapping("name", 3, FieldMapperColumnDefinition.<JdbcColumnKey>identity()) but I don't actually understand how I can use that..

[Arnaud Roger]

you can add alias if the column in the db don't match the exact property path

JdbcFactory.newInstance().addAlias("internal_salary", "salary")

would that solve the problem?

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[Marcel Overdijk]

Yes that worked!

I had to add the alias before the discriminator.

It's looking good so far, next step is to include those employees as part of a query retrieving departments. 

Reading https://arnaudroger.github.io/blog/2017/02/27/jooq-one-to-many.html now :-)

[Marcel Overdijk]

Is it possible to use (nested) discriminator classes like:

JdbcMapper<Department> departmentJdbcMapper = JdbcMapperFactory

        .newInstance()

        .keys("id", "employee_id")

        .<Employee>newDiscriminator("type")

        .when("Internal", InternalEmployee.class)

        .when("External", ExternalEmployee.class)

        .mapper(); // returns a JdbcMapper<Employee> instead of a JdbcMapper<Department>

The problem is that the mapper() method returns the nested discriminator class instead of the top class?

[Arnaud Roger]

It’s not possible at the minute, would need to have a look at how I can do that in theory it might be possible. Though I’m in the middle of rewriting all the csv mapping to fit in the regular model so would not happen just now

Alternatively a solution might to use a builder for your employee that would instantiate the right class. If you add a static builder method on the root type Sfm should pick it up

Sent from my iPhone

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[Marcel Overdijk]

Is it possible to have the builder outisde of the root type itself? (I cannot modify the root type class).