Fixing Duplicate Data from Hibernate Queries

How to Fix Duplicate Data from Hibernate Queries

This problem was the bane of my existence when I first started using Hibernate because I had no idea where the problem was coming from.

If you're executing a query and finding that you have a bunch of duplicate records and haven't a clue why, then you're in the right place.

You see the problem is typically caused by having left joins (or optional joins) in your objects. When you have a base object, like say User and it joins to another table/object in an optional One-to-Many or optional Many-to-Many format, then you may get duplicates.

Consider this scenario… A User objects joins to the LoginHistory object, which keeps track of all the times a particularUser has logged into the system. And let's say our user has logged in many times. You'll have a situation where you have many records in the LoginHistory table.

So what happens when you run a query that joins to the LoginHistory table? Well it will return as many rows as there are entries for that User in the LoginHistory table.

So because of this, Hibernate doesn't massage the data for you, it just returns exactly what it got from the database. The ball is in your court to tell Hibernate what to do with records it has retrieved.

There are two solutions to this problem:

1. Declare your joining object as a Set
2. Make use of Distinct Root Entity Results Transformer

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