Add Query by Example feature #2418

[ezequielantunez]

Added Query by Example repositories fragments
Added Criteria.regexp

Closes #2418

[sothawo]

Sorry I didn't comment earlier, thanks for the PR. I had no time yet to look at it (I was busy on #2419). Will have a look these days.

[sothawo]

Impressive work, thanks a lot for that. I have some minor comments, and one place where I think the logic for getting the property name is not correct.

One thing that I don't like - and that has nothing to do with your implementation - is that by implementing org.springframework.data.repository.query.QueryByExampleExecutor we have to do this unwrapping in the methods of QueryByExampleElasticsearchExecutor; we discard all the additional information that is available in the SearchHit<T> objects.

This could be solved by creating a new interface SearchHitsQueryByExampleExecutor (I don't have a better name currently) where we have the same methods, but different return types (SearchHits<T> or maybe List<SearchHit<T>>). The implementation for that would be similar to what you did but without the unwrapping.

Maybe we could have both, and the unwrapping implementation might use the other one to get the SearchHits and then unwrap them.

What do you think about that? We can merge your changes and have that other as an additional feature later.

[sothawo]

This could be solved by creating a new interface SearchHitsQueryByExampleExecutor (I don't have a better name currently) where we have the same methods, but different return types (SearchHits<T> or maybe List<SearchHit<T>>). The implementation for that would be similar to what you did but without the unwrapping.

In addition this would allow to have method overloads that not only accept a Sort, but a Pageable as well.

[ezequielantunez]

Comments solved in bbadfe9

About creating a new interface I think we could work on it on another PR.
In fact we could have a base repository fragment implementation for Query By Example returning SearchHit, and refactor the fragments included in this PR to use the potentially new SearchHit implementation and just unwrap the results as you mentioned before.