Is Entity Inheritance Possible with a single Repository?

[pluttrell]

Is there a way to use a single Repository to produce multiple types that inherit from an abstract class if they are stored in a single table? For example if Cat and Dog both extend Animal and are both stored in a table called Animals? The single Repository would need instantiate the proper type when returning instances.

JPA supports this with @Inheritance(strategy = SINGLE_TABLE), @DiscriminatorColumn and @DiscriminatorValue as shown here: https://saurabh1.com/2015/04/14/inheritance/. I've tried this example but it fails with this error:

Caused by: java.lang.InstantiationException: example.Animal
	at java.lang.Class.newInstance(Class.java:427)
	at com.amazonaws.services.dynamodbv2.datamodeling.StandardBeanProperties$DeclaringReflect.newInstance(StandardBeanProperties.java:183)
	... 34 more
Caused by: java.lang.NoSuchMethodException: example.Animal.<init>()

[derjust]

Right now it does not support this - the mapping is done by DynamoDBMapper and it does not support this; and it does not support passing in an instance object to get filled.

I can see how this get implemented but will be quite expensive:

• The load of an instance would require 2 loads (one for the key and the type, one for the real thing)
  • And I don't know if the mapper meta data cache support that (same table, different columns/type)
• Scan operations would be hard because you could only serach for animal - or pass the type in for each of the find methods - as the Repository would be typed to the super type.
  • Worth checking how the JPA guys do it :)

So a lot of work, theoretically possible (and therefore quite a long way down the road)

[pluttrell]

I discovered that others are looking for this as well and there's an open PR from about 7 months ago.

[derjust]

Thanks for digging that up. Worth a shot to build a custom AWS SDK and Test it out Thanks! On Feb 2, 2018, at 00:57, Peter Luttrell <notifications@github.com<mailto:notifications@github.com>> wrote: I discovered that others<aws/aws-sdk-java#832> are looking for this as well and there's an open PR<aws/aws-sdk-java#1229> from about 7 months ago. — You are receiving this because you commented. Reply to this email directly, view it on GitHub<#126 (comment)>, or mute the thread<https://github.com/notifications/unsubscribe-auth/AB9U6aOab0sby5WUyk9ZFk-Vq2diHGNwks5tQqPkgaJpZM4RvsLv>.

[snakeru]

@derjust this could be implemented much cheaper if we store the deserialization hint as the prefix of the search key. See aws/aws-sdk-java#1867